Some of the youngsters were only five or six years old when Julius W. Hobson Sr., the late city councilman and civil rights activist, died of cancer in 1977. But they all joined last night in a happy celebration of his memory with poetry, song and dance at the Southeast school that now bears his name.

About 400 people, including Hobson's widow, his mother, his children, Mayor Marion Barry Jr. and Del. Walter Fauntroy, attended the dedication of the Hobson Middle School for fifth to eighth graders at 12th and E streets SE. It is a school that parents and teachers in that community had planned for three years to provide an alternative to the traditional junior high school program.

But it was also an evening for remembering the fiery, yet compassionate Hobson, a man who made his mark on Washington long before many of the young people in attendance last night realized who he was or what his goals were.

In the stuffy school auditorium, jammed to the point of standing-room only, the youngsters listened wide-eyed as School Board President David H. Eaton, a close friend of Hobson's, told them the kind of stories about his friend that brought home to them what he had done for his town, its residents--and the residents to come who would eventually learn at the school named for him.

The students themselves put together their own remembrance of Hobson--a slide show that recalled how he had led 80 pickets in front of 120 major chain stores in Washington that did not hire blacks, got transit officials to hire more black bus drivers and sued the school system to prevent practices that he felt were giving more services to students in white schools than in black schools.

Student Victor Bennett, 12, said he was fascinated most by the story of how Hobson once threatened to throw rats on doorsteps in Georgetown to dramatize the housing plight and deplorable sanitary conditions in some black neighborhoods.

"I was amazed to learn he had run for vice president in a minor party campaign in the 1960s ," said James Galmore, 11. "I didn't know he had done so much stuff."

"He believed in equal rights for blacks and whites; he wasn't a prejudiced man," said Eric Poole, 13.

Hobson's widow, Tina, who watched the two-hour ceremony, her eyes occasionally brimming with tears, said it was "the children" who had impressed her the most. She said "Julius would have loved" the fact that a school that had so much community support would be named for him.

She presented the school with two engravings that she said were among Hobson's favorites. His mother, Irma Hobson Reynolds, gave the school the plaque that the Howard Law School Bar Association had given him in 1968.