Santa Claus got a surprise assist from First Lady Nancy Reagan yesterday when she helped distribute "purchases" to handicapped and needy children who shopped at the Information Center for Handicapped Individuals' "Christmas Store."

"I hope you all have your lists for what you want to buy with all that marvelous money that we wish was real," Mrs. Reagan told more than 5,000 youngsters who had gathered with their families in the cafeteria of the Brookland Elementary School in Northeast.

She greeted a number of children confined to wheelchairs, and then proceeded to hand out plastic airplanes and rockets, baby dolls and toy trucks to the eager customers. Each had received $10 in play money with which to purchase the new and used donated toys.

"I think it was really nice of Mrs. Reagan to come and see the kids," said Stanley Hunter, a warehouse supervisor from Capitol Hill, whose daughters Sacajunea and Kanisha got to shake the first lady's hand.

Mayor Marion Barry and City Council member Charlene Drew Jarvis also helped to give out the toys.

But it was the toys, not the dignitaries, that brightened the eyes of the child shoppers. Eight-year-old Dwan Fitzhugh bought a bicycle with his $10. It was black and chrome-colored, and stood almost as tall as Dwan himself. He fell in love with it the minute he saw it because of the basket on the back that he said he can use when his mother sends him to the store.

"I'm just thrilled," said Joan Fitzhugh, Dwan's mother. She said she has 13 children and was able to get bikes for three of them at the Christmas Store.

"There was no way I would have been able to do this otherwise , because my husband is out of work," she said.

This is the 12th year that the Information Center, an organization concerned with the rights of the handicapped, has sponsored the "Christmas Store." The event has grown from serving a few hundred children in 1971 to more than 5,000 this year.

With that many youngsters and their parents, the Brookland school's cafeteria looked and sounded as hectic as Woodward & Lothrop on Christmas Eve.

And, as in the big department stores, a dollar did not buy as much. In previous years, the children could get some toys for $1 in play money. This year, the cheapest toy was $3.

Bernice Muldrow, 12, from Brookland, got a Pac-Man puzzle and "Crazy Eyes" pop-out eyeglasses. Jodante Fergusen, 7, from Columbia Heights, got a Smokey Mountain Line train set that he said he was going to give to his brother. And Gregory Johnson, 7, who lives in Brookland, spent his $10 on a huge stuffed buffalo "because I liked it."