No," she said heading across the room to the grand piano, she didn't think she wanted to do it without her "costar."

"You know how it is when you're trying to help someone get started," joked Nancy Reagan.

And, admittedly, songwriter Joe Raposo wasn't Frank Sinatra, who sang with her a week ago. Neither was the Lincoln Center library's main gallery the White House. But the Nancy Reagan "To Love a Child" show went on anyway tonight. It could hardly have been a more enraptured audience that publisher Bobbs-Merrill brought together for the launching of the first lady's new book about one of her favorite causes.

And with 300 listening in, the first lady couldn't resist joining Raposo on the song of the same title that he and lyricist Hal David wrote to help publicize the book.

Raposo played it again for her to exit on, which she did with a little soft shoe, blowing him a kiss on the way out.

Like any author plugging a new book, Nancy Reagan was on the road and looking for buyers today. She did this with some well-targeted media events involving children and the elderly, areas in which she happens to be the administration's most effective spokeswoman.

And last night she wrapped the day up at the piano in the duet with Raposo.

"Of course, I know you're all going to go out and tell everybody what a marvelous Christmas present the book would make," she told the preparty gathering of journalists and book reviewers. Then laughing, she added, "You see, I've learned about these things quickly. Somebody whispered that to me."

She learned for the first time tonight that the book already has been bought for partial dramatization on TV.

"Including ancillary rights of serialization by Family Circle magazine, and syndication, $100,000 has already been raised for the Foster Grandparents program," said Sheila Tate, Mrs. Reagan's press secretary. Under negotiation are paperback and publishing rights. All profits from the book and the song, which has been recorded by Sinatra, will go to the program.

The guest list included two of the first lady's fashion designers, Bill Blass and Oscar de la Renta, both of whom got big hugs and kisses when they made their way through the crowd that surrounded her. Others were Pat Buckley, wife of columnist William F. Buckley and sometime traveling companion of Mrs. Reagan; and socialite Nan Kempner, who said the first lady makes her emotional. "I get all weepy-eyed," Kempner said, "she's such a super dame."

Mrs. Reagan's collaborator on the book, Jane Wilkie, was telling CBS correspondent Mike Wallace about being interviewed by Mrs. Reagan for the job.

"She took off her shoes after three minutes and tucked her feet under the cushions. I said to myself 'Thank you, God,' 'cause I knew I'd passed muster," said Wilkie.

ITT chief executive officer Rand V. Araskog couldn't have been more pleased that Bobbs-Merrill, a subsidiary of ITT, had been selected to publish Mrs. Reagan's book.

"We're going to do everything we can, because the more successful it is, the more money is going to go into the Foster Grandparent program," said Araskog.

Grace Shaw, publisher of Bobbs-Merrill, confessed to being a little nervous earlier in the evening when she introduced Mrs. Reagan to the media audience. But by the time everybody was upstairs at the party, Shaw had relaxed. Her firm had spared no expense for food, with several buffet tables laden with smoked salmon, roast beef and baked ham, much of which went uneaten.

The setting was the inspiration of Joan Canole, administrative associate of the library and Lincoln Center Museum. It featured an exhibit from the forthcoming movie "Dark Crystal," by muppet originator Jim Henson.

Mrs. Reagan began the day by spotlighting her interests and support of the Foster Grandparents program with a visit to the Babies Hospital division of The Presbyterian Hospital at Columbia-Presbyterian Medical Center.

She signed autographs, posed for camera buffs and made some unscheduled stops at children's bedsides that even her highly protective Secret Service agents couldn't turn down.

"I don't know how she does it. She sees kids in the most heartbreaking condition," said her staff director, James Rosebush, as he watched the first lady talk to a young kidney transplant patient and his donor.

Mrs. Reagan visited a model program at the hospital that provides treatment and crisis intervention for abused and neglected children. A year ago, foster grandparents were brought in as go-betweens for parents and children undergoing therapy.

"It's a unique unit and needed desperately to educate not only the children, but the parents -- sometimes as young as 16 and 17 years old -- trying to cope with things they don't know how to handle," said Dr. Felix E. Demartini, president of The Presbyterian Hospital.

One young mother, whom the hospital identified as Iris, said her child's foster grandparent "has more patience with him than I do."

Replied Arlene Jerome, the foster grandparent: "I just help them learn how to cope by giving them lots of love and understanding."

Mrs. Reagan sat between Iris and her 2 1/2-year-old son, Luis, watching him draw. When the first lady added a colored sticker to the drawing, Iris told Luis not to "mess it up. I want that for your wall."

Mrs. Reagan also visited the hospital's pediatric recreational therapy program, where the foster grandparents gave her a red smock like theirs. She hugged the babies and offered words of encouragement to the older children recovering from major illnesses or surgery.

"You're going to live a long life," she told Paul Sovany, 14, whose leg was amputated three months ago and who underwent lung surgery last week. He has bone cancer.

At a reception, also at the hospital, with many of New York City's 180 foster grandparents, Mrs. Reagan told them that even if they weren't in her book, "the book is about you."

Janet Sainer, commissioner of New York City's Department of the Aging, said Mrs. Reagan's interest in the program "highlights an image of aging, that older people can do things, and will do."

Sainer said the Foster Grandparents program in New York receives about $500,000 annually in federal funds.

She said she thinks Mrs. Reagan's influence has been significant. "I think her interest is a very important one, and we look forward to her continued support."

In an interview taped Tuesday at the White House and broadcast this morning on the CBS Morning News, Mrs. Reagan told anchorwoman Diane Sawyer, "I love kids and I think that for a long time we were so youth oriented that we had forgotten about the elderly who have a great deal to give."

She said she became interested in the Foster Grandparents program when her husband was governor of California. The program is federally funded by ACTION.

Coordinators of the program, who have asked for $48.4 million for fiscal year 1983, are hopeful that unlike other social programs it will excape budget cuts when they are voted on later this year. How did she "weigh" whether the Foster Grandparents program was the one that should continue, Sawyer asked Mrs. Reagan.

"I think it has been cut to a certain degree," Mrs. Reagan said, "and I think it's been cut according to the states. An ACTION spokeswoman in Washington said later that these "cuts" were regional and were part of the $2.4 million that the Office of Management and Budget chopped out in the middle of fiscal year '82. This reduced the funding level to $46 million. Under the continuing resolution, the program is still operating at that level.

"I don't have anything to do with what programs have been cut and what programs haven't been cut. I think all programs have been necessarily cut out of necessity -- What was your question?" Mrs. Reagan asked, with a laugh.

When Sawyer restated it, Mrs. Reagan told her, "Now you're getting into things I couldn't answer . . . I just think this program is a good program. It benefits two groups of people who need help. Each gives to the other what the other needs."