If you have a family, then you know what families out are really like. We took Route 1 toward Woodbridge on a warm Saturday night. We left the District about 7:15 p.m., heading down that particular road because it was the route described to us on the telephone. It was a little late for 2-year-old Sarah to be heading for supper - but never mind, the restaurant was only 20 miles away, we said blithely. That was before we had stopped at a traffic light at every intersection in Alexandria; before Sarah had gotten carsick, requiring a sidetrip into a shopping center for a change of clothes, and before we encountered a massive traffic tie-up just south of the Occoquan River bridge, where both northbound and southbound lanes of traffic were stopped by an auto accident.

It was then we discovered Route 95, a faster way to get to Woodbridge and the only way around the traffic.

We got to the Casa Blanca a little after 10 p.m., but no one looked dismayed to see us walk in that late - they close at 10:30 p.m. The hostess immediately pulled up a high chair for Sarah. We sat down and quickly ordered milk for Sarah and beer for us - Dos Equis, a good Mexican beer. With the milk and beer came tostadas - fried pieces of tortilla - which kept Sarah happy with something to munch while we looked at the menu.The sauce served with the tostadas was hot with chiles and good. We let Sarah have a taste of the sauce but turned her down when she wanted more, mindful of her earlier car-sickness.

My husband ordered the house special - a taste of virtually everything on the menu, for $5. I asked for the Picadillo, a combination of enchilada, chili relleno, rice and beans and a dab of guacamole for $4.15, which is the price of all the combination dinners. Sarah had a child's enchilada plate for $2, which with its enchilada, rice and beans was really enough for an adult and the best buy on the menu.

The menu was standard north-of-Wichita Falls, Tex., fare. The only departures were flour tortillas, offered ala carte - and not offered at all in most area "Mexican" restaurants, and sopapillas for dessert. For those who can't even take the bland northern version of Mexican food, the Casa Blanca offers a Gringo-burger - a basic hamburger for $1.65 or $1.75 with cheese. There are also a Chicano-burger, with spiced meat, a chopped sirloin for $4.95 and barbequed chicken.

The food was fit for Yankees, more bland than hot. The chiles rellenos (peppers stuffed with cheese, coated with batter and the tamales, which looked handmade and tastedhomemade, were the best. Both are available ala carte. Sarah liked her enchilada and beans and shoveled her supper in, first with a fork and then with tostadas.

The Casa Blanca is a pretty place, with white, rough plastered walls to look like whitewashed adobe, decorated with paper flowers and Mexican pottery.The sound system gave us a background of Mexican records. It was a pleasant respite from the trip to Woodbridge. Although we were late and lingered a bit over the last of the food, we weren't pressured to get up and march out. Nobody hung over us whisking dishes away or vacuumed under our feet.

The child's plate comes with dessert - sopapilla, the only desert they have. It is a light piece of fried bread served with honey, a good, light dessert after a somewhat heavy meal.

With four beers and a generous tip - in memory of some of Sarah's supper which had hit the carpeted floor - the tab was $20. Without beer and tip, it would have been $12.43.

The Casa Blanca seems to have been called Casa Bonita in a previous incarnation and is still listed that way with information. They offer all-you-can-eat lunches for $2.99 to $3.75.

If you need to telephone someone while you're out, plan to do it before you arrive or after you leave. There is no pay phone and they claim to have telephone for incoming calls only.

Atmosphere: Restful, with white walls, Mexican pottery and low, but not dim, lights.

Price range: $1.65 for a hamburger and ala carte selections in the $3 range; $4.15 for combination plates and $5 for the house special, which has a taste of almost everything.

Hours: 11 a.m. to 10 p.m. Monday-Thursday; 11 a.m.-10 p.m. Friday and Saturday, and noon to 10 p.m. Sunday.

Special facilities: Patrons in wheelchairs accommodated, although there is a step up from the parking lot in front of the restaurant.

Reservation: Not necessary except for large parties. Credit Cards: Master Charge, Visa, Bank Americard and personal checks.