Atmosphere: Information and festive.

Price Range: From 95-cent beans to $7.25 steaks.

Hours: Mondays through Fridays from 11 a.m. to 11 p.m., Saturdays from noon until 11 p.m. and Sundays from 2 p.m. to 10 p.m.

Special Facilities: Accessible by wheelchair. There are high chairs, booster seats and children's portions. Limited street parking; basement garage involves a fee.

Reservations: Not necessary

Credit Cards: Americans Express, Bank Americard, Master Charge.

In our family, where "south of the border" could easily mean somewhere in Prince George's County, Mexican cooking is not the most familiar happening. But as it happens, we got down to brass tacos the other evening with a visit to the The Maria down along Washington's Maine Avenue waterfront.

Even before tasting at thing, the jaw drops at the decor. It's about as dull as a mouthful entranceway through the various friendly rooms of this rambling hacienda, we follow our beatifully costumed hostess past all manner of hanging s- from old, crude farm implements, to coppers pots, oil paintings, bullfight posters, colorful pottery, books and - to top it off for this holiday season - paper chains and a jolly pinata.

Each table sports a mixture of red, green and white napkins and, as they say in Guadalajara, hark - there is live music: four guitars, a violin and a horn. The musicians are playing, among other things, old Mexican favorites such as "Jingle Bells" and "Happy Birthday."

As the musicians meander along to serenade an empty barroom, we are seated in four of the many different straight-back chairs at a table with a fine view of the lighted boats gliding by.

For the children, (and there are quite a few of them here on this paricular evening), this is the closest thing to a G-rated night club that a family could find.

The first required reading is our waitress, who wears a nameplate that says "Cherryl" and a button that says "Stamp out Gringo Food/Try Tia Maria."

Next there is the menu, which, when opened up, is the size of a small door. On one side is a mini-narrative on Mexican history and food that tells you, for example, that "prior to the coming of the Spaniards, the Indians had no knowledge of fats or oils, and thus no knowledge of the art of frying. The Conquistadores brought with them their lard-fatted pigs, and a whole new culinary technique was born."

Well, ole - and on to two Cokes in grand, nine-inch-tall glasses, a glass of sangria, and a generous margarita ("tequila, triple see and tangy lime. . served in a frosty salt-rimmed glass"). You needen't bother to order an appetizer, for a basket of thin and snappy corn chips accompanying by hot sauce will arrive to fill that need.

For parents, the operative words on the menu are in the lower right-hand corner: "para los Ninos," which is the list for children under 12. For $2.50, there is a choice of cheese enchilada, crispy beef taco or chopped sirloin, each served with salad and a choice of Spanish rice, refried beans or fried potatoes.

With our ninos the beef taco wins out twice, once with potatoes for your 10-year-old son and again with surprisingly good beans for our 8-year-old daughter. Their portions are anything but skimpy and, like ours, are artfully arrranged on large platters.

For my wife, an instant hit is the enchilada de crab at $5.95 - two enchiladas stuffed with huge portions of crab and covered with mushroom sauce, cheese with mushroom sauce, cheese and a dash of sour cream, accompanied by a salad and Spanish rice that's nothing like the kind that camps and schools always serve. I tackled an enchilada offering entitled Acapulco, at $5.25. It's billed as being for those would just as soon not have to make a difficult decision, and that had to be me. It's a combination of one enchilada filled with chickens and another with cheese, and I still haven't decided which one I liked better.

The Tia Maria also tosses in a basket of hot tortillas with lots of butter and hot sauce, which, the menu tells us, were the staple food fo the Aztecs long before the coming of the Conquistadores (who apparently came without reservations and were seated somewhere).

Incidentally, for culinary scaredycats there is chopped sirloin with French fries and a lettuce and tomato salad for $4.50, and fried shrimp with the same accompaniment for $5.95. There are steaks, too, at $6.95 or $7.25.

Beyond that, there are several dozen dishes in the $5-to-$6 range, all helpfully described for the novice.

The only taker for dessert was our son, who sampled the vanilla ice cream with cherry sauce for 95 cents while the parents enjoyed mugs of coffee.

The bill, which arrived with four little wrapped toffees, came to $23.76, plus tax. In today's world of high-note prices, that' was music to our ears. An encore is in store in 1978.