Some parents can hardly wait to send the kids away to camp. We can hardly wait to join them there.

We discovered the family sessions at Camp Letts, the YMCA facility in Edgewater, Md., several years ago and with our four children have been regulars there ever since, doing the whole Jolly Camper bit - "First Period: Archery Tournaments; Second Period: Sailing Lessons; Third Period: Five-mile Hike; Fourth Period: Swimming . . ."

Other families must share our enthusiasm because the Memorial Day weekend session is fully booked (though families can still attend if they bring their own tents or campers) and the sessions for Labor Day weekend and the week before is fast filling up.

Of course, the more makes it all merrier, especially the highlight of each weekend: the sailing regatta, with prizes awarded for every category necessary so everyone gets one - completing the course, getting to the starting line, and in one famous case, sailing backwards. A Camp Letts tradition that never fails is that in the middle of the regatta the wind dies to nothing. Then there's a great chorus from the shore: "Get out and push!"

If you're mal de mer prone or otherwise a non-sailor, Camp Letts has loads of other diversions, including tennis, horseback riding, volleyball, rowboating and canoeing, softball . . . simply sunbathing by the pool or even curling up with a book.

But we're strong on the organized events like the canoe expeditions down the river at 6 a.m. culminating with a pancake and sausage breakfast cooked on huge outdoor grills. Or the evening square dance. At the one last fall we wore out the caller.

"Where do you people get your energy?" he finally shouted. "It's 98 degrees in here, you've been stomping and skipping and clapping for three hours, there are two children sound asleep on the table, I'm exhausted, and you're all still rarin' to go."

It's true. No one has time to get tired, and no one notices stiffening muscles, sunburn and mosquito bites until departure time late Monday.

Rainy days are the bane of camp directors and Ambery Butcher of Camp Letts is no exception. At every breakfast he exhorts us that if our daily rendition of "Zippadee DooDah" is done "loudly and with spirit," the sun will shine all day. The singing is deafening.

On the exceptional day when the chorus fails, collective ingenuity comes to the fore. Card and table games are set up, crayon and clay projects are started, indoor relay races are organized, a brave soul sets out for newspapers, after one deluge a boat-bailing contest was held.

Perhaps the No. 1 activity at Camp Letts is eating, always family style, with mothers especially appreciating the vacation from meal planning and preparation. Nothing epicurean - fried chicken, roast beef with gravy, hamburgers, fruit salad, mashed potatoes, bacon and eggs - but always plenty for second, even thirds. The coffee pot is always perking and for the children milk is always available, or Camp Letts' famous "bug juice" - fruit punch. There's even a snack session evenings at 10 with sandwiches, tuna salad, cake.

Often there's an evening crab feast with bushels of hard shells, Butcher's own chowder, corn on the cob. For non-crab lovers, hot dogs.FAMILY CAMP SESSIONS

Camp Letts accomodations are cabins with bathrooms in separate building or rooms in the lodge. Fees for three-day weekend sessions (horseback riding is the only extra charge) are $45 for adults 14 and over, $25 for children 9-13, $15 for children 4-8. For the entire week preceding Labor Day, add about $30 per person. For details, reservations, call 261-4286.

In summer, Camp Letts is filled with kids 8 through 16. In the off-months, the facilities are available for groups such as Scouts, schools, training sessions, retreats, meetings.