Fifth and sixth graders of Stevens Elementary School took a walk around Pennsylvania Avenue yesterday to look at buildings.

"I like buildings best when they go all different ways," said Amy Carter, who lives on the avenue in a colonial mansion with columns and pediments.

The walking tour started in front of the Martin Luther King Library at 10th and G streets NW. Amy's group of 10 children (there were 30 in all) was led by Nancy Muller, a trained guide of ArchiTours, the non rift education organization which has been showing off Washington's architecture since 1976. The Critic trotted along.

Muller got her group to huddle and, with eyes closer, imagine an old-fashioned market with small houses and horse carts and people selling and buying things and meeting each other. Then they all looked up and - boy, has the city changed.

But when the children got through discussing what had happened - what with cars rushing around and buildings growing taller - they decided the city was still a market-place. People come downtown to shop...and to eat...and to see shows and museums..."and to sit in parks," said a girl whose name is Ursula. (Smart kid, thought Critic.)

Then Muller gave everyone a sheet of paper with either a square, or an arch or a triangle on it. The children looked for these forms as they looked at these buildings.

The Martin Luther King Library was all square, Muller explained that its architect Mies Van der Rohe, believed that "less is more." She also explained that the library was designed so that it could grow one more storey higher if needed. The group decided it liked low buildings better than high buildings. (Smart kids, thought the critic.)

Turning around, the group studied St. Patrick's Academy. They liked all the arches and all the other forms on the facade. Amy pointed out the small steeple. Another youngster said that the rough stone made the building look strong. Muller said it was built about 1890. "As old as my grandmother!" someone giggled.

Muller explained about "the human scale and what that meant. The group agreed that people should feel comfortable with their buildings. A cuilding should not seem scary and overpowering. (Smart Nancy Muller, thought the Critic.)

By that time the group was standing on E. Street and looking at the FBI Building. It did not seem scary and overpowering to any of the kids. Amy said it wasn't at all too tall, "just very wide."

Muller asked, not without a tinge of bias, "Would you want to go inside, or does it seem too forbidding?"

"Inside, inside!" everyone shouted.

(Never trust a fifth or sixth grader, thought the Critic.)

Muller, not one to be easily defeated, asked. "What does the FBI Building make you feel?"

"Safe," said a girl named Marshal as quick as a G-man's pistol shot.

(Score one for J. Edgar, mused the Critic sadly,)

Amy took copious notes with a neat, even hand and, from what the Critic could indiscreetly glean, in short, clear sentences. When a page was full, she tore it from the spiral-bound notebook and put it in her tote bag.

Even if she seemed fully absorbed with her note taking, Amy's was the first hand up whenever a question was asked.

Before Pat Coan, the Stevens School teacher in charge, marched everyone back onto the bus, they discussed the city some more. They obviously like their city. What troubled some children was to see Lansburgh's Department store vacant and boarded up. (What pleased the Critic was that at last children are being taught to think about our surroundings.)

ArchiTour's school program for childre, "Learning to Look," has been introduced in seven schools as part of a D.C. history course. ArchiTours is supported by the National Endowment for the Arts and the Morris and Gwendolyn Cafritz Foundation.