Nancy Reagan snipped the pink bow on her "best" birthday present yesterday, and watched as two children eased into the pool some of her friends refurbished in her honor.

The indoor swimming pool at the D. C. Center for Therapeutic Recreation in Southeast, one of three such facilities in the District, required sandblasting and painting, all at a cost of $3,800. According to publicist Robert Keith Gray, 18 donors, including Bendix vice president Nancy Reynolds and former ambassador Walter Annenberg, have raised "something more than that" and will donate any further funds to the recreation department.

Mayor Marion Barry addressed the first lady and the crowd, saying, "If you have some friends who want to give some more, they're certainly welcome to."

Nancy Reagan blew out the candles on a two-tiered birthday cake but would not reveal her wish: "If I tell you, it won't come true," she said. The cake was decorated with a swimming pool made of blue-green icing which began to ooze under the television lights. Said one CBS cameraman: "I've got enough shots of the cake; when do I get on line for my piece?"

After the ceremony, children from D. C. General Hospital were helped into the pool for an afternoon of swimming and water basketball. At one point, the first lady threw a huge beach ball into the water, boppin one of the volunteers on the head. Both waved to each other and laughed.

Of the new paint job on the 4-year-old pool, Department of Recreation head William H. Rumsey said, "While we couldn't have first-lady red, we did get first-lady white and green."

"This is the best present I've gotten," Nancy Reagan said.

"I'm a big believer in voluntary giving," she added. "I'm sure there will be lots more, not only in Washington but in all the other places."

Speaking of giving, the first lady's press secretary Sheila Tate said yesterday afternoon that Nancy Reagan had yet to receive a gift from the president. "There's nothing yet. We're getting worried," she said, laughing. "But I assume that will come tonight."