IN SPRING a young person's fancy turns to theme parks, and the younger the person the fancier the fancy. As of next Saturday all the parks in this region will be open, at least on weekends; here's what they cost, how to get there and some of what to expect:

WILD WORLD

Washington's closest and least expensive park, which is in some ways also the best, has been having tough times. Last year Wild World tried cost-cutting measures that included trimming the payroll and some of the rides, closing a kiddie section and eliminating the animals that gave the place its name.

Patrons complained and stayed away in droves, and the weather didn't help. This year, the park's trying to turn things around by expanding, restoring some rides and beefing up its show schedule. This has led neighbors to complain about noise and traffic, which has caused the Prince George's County Council to caution the park managers to take it easy.

But are they downhearted?

"No!" says Lyle Wolinsky, one of the head honchos. "We'll just have to try harder and do better. The weather killed us last year; we have to give people more to do when it's too cold or rainy to go in the water. And we're going to have to add major attractions like roller coasters."

But for the time being the best thing about Wild World remains its Water Works section, and that is very good indeed. Besides Wild Wave, billed as the world's largest pool, there are eight (8!) water slides, a vast kiddie pool and two acres of carpeted sundeck.

The food, which wasn't bad, should be better yet under the direction of a Cajun-trained chef who plainly can't resist his own creations.

Back again is Maxwell's Great American High Dive Show, featuring world champion Patti Davis. New shows include puppeteer Darrell Osher, who is a delight; and "Broadway Carousel," created and performed by Wild World's own company.

The least tangible yet perhaps most important asset of the park has been the cheerful competence of its mostly young and mostly local staff, whose morale slumped under last year's penny-pinching; Wild World's fans are hoping the park has put this pound-foolishness behind it.

WILD WORLD -- Half an hour from downtown Washington at 13710 Central Ave., Mitchellville, Md. (three miles beyond the Capital Centre). From the Beltway, take exit 15A. By Metro: Blue Line to Addison Road and a T10 or T11 bus. Open May 18 and 19; daily from May 25 through Labor Day, plus weekends in September. Admissions ($2 discount coupons available from Drug Fair): $9.95 adults; $7.95 senior citizens and children four to 10; under four free. Season passes $34.95. Parking $2. 249-1500 (local call in Washington area).

KINGS DOMINION

One major sign of confidence in the future of Wild World is the one that Kigs Dominion recently erected near the rival park's main entrance. It's this sort of hustle and attention to detail that has made Kings Dominion the region's best theme park.

To keep us coming this year, Kings Dominion has added its fourth water flume. Called Diamond Falls, it's a thousand feet long, with a 50-foot drop into an old diamond mine where just about everything but Indiana Jones jumps out at you. In keeping with the park's growing liquidity, the ride gets you pretty wet, although nothing like so wet as Whitewater Canyon.

While the new ride pulls all ages, it's aimed at the tweenagers who're a little too big for the kiddie stuff but can't quite make the height limit for such big-people rides as the gutwrenching Grizzly roller-coaster and the lunch-launching Berserker boat, which will hang you upside-down six stories in the air, if you're so minded.

Also new this season is Scooby's Olympic Park, which has a lot of tunnels, nooks, rannies and ball-crawls for kids three to 10. Best of all, it has lots of shaded benches where mom and dad can cool out for the half-hour or so that the equipment can be expected to keep their little scarpers scurrying. Kings Dominion now has 40 major rides; three featured shows, including "Gotta Dance," a new Broadway-style revue; a 330- foot replica of the Eiffel Tower from which, on a clear day, you can see Richmond munching through the piney woods as it marches inexorably toward Washington; an antique wooden merry-go-round; an animal park and a steam train. The Grizzly and the twin-track Rebel Yell are classic wooden roller-coasters that draw buffs from all over the world.

Beautifully landscaped and maintained, the park is sufficiently spacious that it doesn't seem all that crowded although it always is.

KINGS DOMINION -- Doswell, Virginia, 75 miles south of Washington on I-95. Open weekends through Memorial Day; daily through August (plus Labor Day); weekends through October 12. Admissions: $14.50; 65 and older, $10; children up to three, free. Parking $2. 804/876-5000.

BUSCH GARDENS

It isn't any bigger, but the Big Bad Wolf is better this year, and should satisfy those masochists who found the $6 million ride too tame when it opened in midseason 1984.

What they've done is to put up more things that the rocketing ride almost runs into, and move them in a little closer, the better to glee you with, my dear. What with all the up-and- down and rocking around the BB Wolf does anyway, it makes a fitting companion to the double-loop Loch Ness Monster.

The only complaint a motion-sickness fan can make, and it is a churlish one, is that both of Busch Garden's major thrill rides are too well-made: The action's so smooth and precise, so lacking in rickety-rackety, that even in the moments of greatest gravity you know you'll come back alive.

Although many Washingtonians daytrip it to Busch Gardens, that makes an awful rdeal of it, especially for the driver. A better idea is to make a weekend of it, staying at nearby Yorktown, Williamsburg, Virginia Beach or Norfolk, each of which has plenty of other attractions that will make the family glad they came. The park has reduced-price two-day admissions also; you may find it takes two days to hit all the high spots, the lines are that long.

BUSCH GARDENS -- Williamsburg, Virginia, 160 miles from Washington via I-95 south and I-64 east. Superhighway haters may want to take the U.S. 17 bypass at Fredericksburg, which takes you through enticing Tappahannock Town and salubrious Saluda. (The charms of both towns are rather well concealed from the main drag, but explore a little. This will make you run late, but that's fine, because it will be time to eat when you reach Yorktown and Nick's Famous Seafood Pavilion.) Park open daily through Labor Day and weekends through October. Admissions: $14.95; children under three, free; two-day tickets, $17.95; nighttime tickets $9.95; season passes, $49.95. Parking $2. For information: 522-1387 (free call from Washington area).

HERSHEY PARK

If you want to see what the Glen Echo or Marshall Hall amusement parks might have been, if only Our Town had had a rich and benevolent Milton S. Hershey, visit Hershey Park, which the chocolate king opened for his workers in 1907.

Visit Hershey Park anyway, and to hell with the history, because it's a delightful place that still seems cozy and laidback although it now has 39 rides, including three roller-coasters, plus five theaters and seven restaurants.

ZooAmerica, reached by a covered bridge, is a compact gem of a place devoted to our own native fauna. While it's entertaining, it's also a serious place where they breed threatened species such as the indigo snake and golden eagle.

Whether it's the beauty of the park, or the solid sense of being somewhere that has been a somewhere since before you were born, or the trace of chocolate scent that's always in the air, or the infectious Pennsylvania Dutch hospitality, or all of the above, Hershey Park's a great place to spend a day. Don't spoil it by daytripping; make it an overnight and check out the visitor center, museum and gardens.

HERSHEY PARK -- 140 miles from Washington via I-270 to Frederick, U.S. 15 north to Harrisburg, and Pennsylvania 322 east to Hershey. Open weekends through May 19 and daily from May 23 through September. Admissions: ages nine through 61, $14.50; four through eight, $11.50; over 61, $8; under four free. Two-day tickets $21 and $17; purchase of a regular or junior ticket after 7:30 entitles patron to free admission the following day. Free parking. 717/534-3900. For accommodations in Hershey, call 717/534-3916.

SESAME PLACE

Where in the heck is Langhorne, Pennsylvania? Well, if you have prepubescent persons in your custody, you need to know that it's the home of Sesame Place, a very small but utterly engaging park hat's owned by the people who make Busch beer but run by the people who make "Sesame Street."

It used to be just an elaborate playground sort of stuck out in back of a shopping mall, but Sesame Place has burgeoned into a water wonderland as well as a place where kids are encouraged to exercise their minds and bodies without once suspecting that what they're doing is good for them.

Until this year most Washington area families that visited the park stopped by on the way to or from vacations; daytripping it north of Philadelphia was just too much. But now you can take the "Sesame Place Express," an Amtrak tour package that includes round-trip railfare from Washington or Baltimore, shuttle service from and to the nearby Trenton station, and park admission. After you explain to the kids what a train is, it should be quite an adventure.

Take along a big plastic bag for the wet swimsuits and towels, because the waterslides and play fountains are irresistible. There are other things to do outdoors, including climbing and clowning -- parents are permitted to play too -- and lots of electronics indoors. And everybody will learn something, if they don't watch out.

SESAME PLACE -- Langhorne, Pennsylvania. AFTER you get north of Philadelphia on I-95, take the U.S. 1 south exit. Look for the Oxford Valley Mall; if you come to a restaurant with a Superconstellation airliner on its roof, you've passed Sesame Place. Park open daily through October 13. Admissions: $9.90 children under 13 (under three, free), $7.70 adults. 215/752-7070 or 752-4900.

SESAME PLACE EXPRESS -- Roundtrip fare from Washington/Baltimore of $58.30 for adults and $36.80 for children includes shuttle bus to and from Trenton station and park admission. Call Amtrak Tours at 1-800-USA- RAIL for information and reservations, which must be made 24 hours in advance.

GREAT ADVENTURE

Jackson, New Jersey, is nowhere in particular, being stuck out there in the Pine Barrens,but it has a McDonalds and the biggest and busiest amusement extravaganza this side of the Disney parks.

The park still is reeling from last May's tragedy, when eight teenagers died in a funhouse fire. Experts have described it as the classic case of everything going wrong at once, but Great Adventure since then has passed a whole host of rigid inspections.

There are more than a hundred rides, including such only-for-the-real-nuts devices as Lightning Loops, which makes your stomach feel like an alien and malign presence; Rolling Thunder, a roller-coaster that seems curiously insubstantial; Parachuter's Perch, the world's tallest ride, which takes you somewhere up near where the clouds form and makes you feel -- like Br'er Rabbit -- at first afraid you're going to fall and then afraid that you never will; and FreeFall, which is the very embodiment of every claustrophobe's absolutely awfullest nightmare about being caged and helpless.

There's plenty for sane and/or grownup persons to do while their children are reveling in these self-tortures, including a drive- through animal park that only costs a dollar extra but is worth the price of admission by itself. There are more than 2,000 animals, all plainly well cared for and roaming as freely as prudence permits. You may park and watch as long as you like, which probably will be quite a long while indeed.

GREAT ADVENTURE -- Jackson, New Jersey, 190 miles from Washington via I-95 to exit 7A on the New Jersey Turnpike to exit 98 on the Garden State Parkway. For an outstanding weekend, drive to the park on a Saturday morning, spend the night at Cape May and Sunday on the beach, and head home via the ferry ($12.50 per car) to Lewes, Delaware and the Bay Bridge. Great Adventure's open this weekend; daily from May 15 through Labor Day; and weekends through October 20, plus October 11. Admissions: $13.95, plus a dollar more for the drive- through safari; $4.95 for safari only; children under free. Parking $2. 201/298-3500.