The parents of a 9-year-old boy who was burned severely by an explosion during a chemistry experiment at Murch Elementary School in Northwest Washington have filed a $20 million lawsuit in D.C. Superior Court against the District government and American University.

The boy, Stewart Ugelow of Northwest Washington, was among four children injured while making handheld fireworks called sparklers as part of a summer enrichment program for gifted students. The two-week program last August, for which parents paid $165 a pupil, was directed by a doctoral candidate at American who was not named as a defendant in the suit.

Officials of both American University and the D.C. school system denied responsbility for the program yesterday. George Margolies, legal counsel for the school system, said the schools "should not be held liable for the acts of somebody who was not under our control." Anita F. Gottlieb, director of university relations for American University, said the summer school was "not our program . . . . They happened to hire a person who happened to have an affiliation with" the university.

The suit said the boy suffered "disfiguring" burns on the face, hands, arms, and elsewhere. It said he underwent "extensive hospitalization" and still requires treatment even though he has returned to school.