It was a serious event, a Rose Garden ceremony to honor exceptional volunteers, that drew the dressed-up group yesterday to the White House.

And it was serious praise that President Reagan delivered as he singled out seven young people, including 10-year-old Monica Perez of the District, for special projects that ranged from the summer school Monica conducted at her home to a Philadelphia youth's work with the homeless.

But 4-year-old Nicholas Speed didn't seem serious at all, and his antics in front of the lectern as the president spoke, at times, completely upstaged the former actor 70 years his senior.

Nicholas, a reporter with Pyramid Communications here and the host of his own "Jump to It" radio show Saturday mornings on WPFW-FM, was one of about 200 youthful volunteers honored by Reagan. But the budding journalist seemed never to let the ceremony or its import overwhelm his need to move about and express his eagerness for it to end.

Monica, a fourth-grader at Nativity Catholic Elementary School, 6008 Georgia Ave. NW, who was chosen for recognition after a Washington Post story about her last summer, was clearly more impressed with the occasion.

"It was exciting," Monica pronounced after her White House visit, on which she was accompanied by her mother Julia Perez and her aunt Barbara Odella.

"He Reagan congratulated me," Monica said, "and he asked me how did I start the school and he asked me how many little children I had and he asked me if my school had a principal."

The girl, who drafted her 13-year-old brother David as principal for her school of 19 students, delivered letters to the White House from her Nativity classmates and followed their instructions to give Reagan "a kiss and hug him."

Representatives of seven local groups that coordinate volunteer efforts for children and youths were also invited to the White House ceremony. They were:*Friend to Friend of Rockville, whose members serve as companions to handicapped youths.*Touch America Project of the District, through which students work on public lands in environmental efforts.*Anacostia Unlimited, which trains youths in technical skills so they may in turn train their peers.*Super Volunteers of the District, which serves as a link among teen-agers in traditional service organizations.*Crime Stoppers of the District, a club for 10- to 12-year-olds that stresses obeying the law and learning self-reliance.*Red Cross Clown Corps of Arlington, whose members entertain at nursing homes and hospitals.*Pyramid Communications, which operates out of the Capital Children's Museum and through which children research, report and produce radio and video programs for worldwide distribution.

Pyramid founder Derrick Johnson said last night that young Nicholas is expected to make a return appearance at the White House today when Reagan honors senior citizen volunteers.

"Nicholas is supposed to interview either the president or Col. Potter [Henry Morgan] of M*A*S*H."