Tramp, tramp, tramp -- the kids are marching: along the corridors of the Senate, down the marble staircases of the House of Representatives, through the underground passages of the U.S. Capitol to the minisubways that link the Capitol to the Rayburn Office Building on the House side and the Senate office buildings.

It is that time of year. Friends, cousins, acquaintances from out of town come to visit. It would not be right to let the youngsters languish at home, swimming, shooting baskets, riding bikes.

They should see something of Washington. What more vivid exhibition of government in action could there be than the U.S. Capitol?

There are majestic rooms with long histories, lively testimony at hearings, droning debates on the floors of Congress, and, of course, the free subway ride.

Eventually, hunger and thirst strike, and it is time for lunch, a snack or a quick refresher where you can rest your feet, study the map and regroup.

When you're visiting Congress you can do as the native staffers and members do: eat at the restaurants, cafeterias, coffee shops and snack bars located in the Capitol or in the office buildings that serve the Capitol. Most are closed to the public during peak lunch hours (11:45 a.m. to 1:30 p.m. on the Senate side), but are open to the public at all other times.

On a recent sunny, relatively cool day I played tour leader for my children and their out-of-town visitors. We wandered into the interior courtyard of the Russell Office Building where an outdoor health salad bar was set up. Open on clear days in June, July and August, the interior courtyard is a soothing and refreshing spot to grab a bite, put up the feet and rest awhile.

The courtyard is graced by a well-groomed expanse of lawn, magnificant rows of bushes and a wonderfully old-fashioned and ornate fountain. A few picnic tables with bright umbrellas were set out on the lawn, but many regulars headed for the lawn itself, where they stretched out and sunned themselves during their lunch hour. Most of the lunch crowd were staff members who worked in the building, but once in a while a senator would appear, down a salad, and disappear.

The salad bar, $2.60 for a dinner-plate serving, is a series of carts packed with ice in which are nested huge bowls of lettuce and spinach plus smaller bowls of interesting toppings: shredded cheese, sliced cucumber, strips of green pepper, mounds of beets, scoops of chopped eggs, quartered tomatoes, cooked apples and raisins, and, the piece de resistance, beautifully seasoned marinated mushrooms.

For another 50 cents you can also enjoy freshly sliced homemade bread or giant oatmeal raisin cookies.The beverages were an interesting selection, catering more to adults than to children. For 75 cents there were small bottles of spiced apple juice, Perrier water or diet soda. From huge, ice cold urns gushed iced tea and lime rickey, 35 cents a shot. The lime rickey was a zesty fruit punch that combined tart lime with mellow pineapple.

A salad lunch is not always a kid-pleaser. Kids tend to freeze when offered hamburger-less, French fry-less lunches out. But those in our little group rallied, heaped their plates with salads and homemade bread, polished off all they took and went back for some giant cookies. The salad greens and toppings were impeccably fresh and well-prepared; the breads were delightfully rich and filling.

While I lingered over my lime rickery, the kids explored the outer reaches of the courtyard and visited the lovely old fountain.

The tab for a scenic and healthy lunch for four children and one-adult was $19.95. Lunch at other cafeterias and restaurants at the Capitol offers a wider selection of foods: sandwiches, macaroni and cheese, chicken, beef and seafood dishes. Complete lunches run about $3 to $4 per person. Eating at the Capitol

On the Senate side, restaurants, cafeterias and coffee shops are closed to the public from noon to 1:30 p.m., unless otherwise noted. On the House side, restaurants and cafeterias are closed to the public between 11:45 a.m. and 1:15 p.m. No reservations, credit cards or checks. All facilities are accessible to the handicapped.

Senate Public Dining Room, U.S. Capitol, ground floor. Stately atmosphere. Open to public 8 a.m. to 4 p.m. Price range: $1.25 to $4.95 for soups, salads, sandwiches and hot platters. High chairs for very young diners.

Senate Snack Bar, in the basement of the Capitol. Busy and casual. 8 a.m. to 4 p.m. Price range: 65 cents to $3 for soups, salads and sandwiches.

Dirksen Office Building cafeteria, Constitution Avenue, across from the Capitol. Big, gray and business-like. 7:30 a.m. to 3:30 p.m. for cafeteria service; 3:30 p.m. to 7 p.m. for carryout service. Price range: $1.95 to $2.50 for hot and cold entrees, sandwiches.

Russell Office Building coffee shop, Constitution Avenue, across from the Capitol. 7:30 a.m. to 3:30 p.m.; stays open later when Senate is in session. Price range: 50 cents to $3 for soups, salad bar, sandwiches.

Russell Office Building interior courtyard salad bar, Constitution Avenue, across from the Capitol. Quiet garden. 11:30 a.m. to 2 p.m., June, July and August. Public restricted when crowded. Price range: $2.60 for salad bar.

House Restaurant, U.S. Capitol, first floor. Stately atmosphere. 8 a.m. to 2:30 p.m. Price range: $2.50 to $5 for hot entrees, salads.

Cafeterias and carryout service at Longworth Office Building, Rayburn Office Building and Annex II. Carryout service at Cannon Office Building. Government-issue cafeteria atmosphere. 8 a.m. to 2:30 p.m. Price range: 50 cents to $2.50 for soups, salads, hot entrees.