IT HAD ALL THE MAKINGS OF A BIG Broadway moment. Legendary baritone John Raitt had just finished belting out the father-to-be's heart-tugging "Soliloquy" from Carousel ("My little girl, pink and white as peaches and cream is she/ My little girl is half again as bright as girls are meant to be") when out walked his own little girl, blues-rocker Bonnie, 42. The occasion, before a crowd of 1,800 at Boston Symphony Hall last week, marked the first-ever public concert by Papa Raitt, 75, and his Grammy-winning daughter. "I anticipated it being incredibly moving, and it was," said Bonnie, who joined her father in singing two of the Broadway show tunes that made him famous ("They Say It's Wonderful," from Annie Get Your Gun and "Hey There," from The Pajama Game). As the evening's finale, Dad switched gears and sang with Bonnie on her 1975 ballad "I'm Blowin' Away." [P] Although the performance (taped to air July 12 as part of the Boston Pops series on PBS) won the duo a sustained standing ovation, both admitted to genre-jumping jitters. "We spent almost three hours the first time we got together, with her telling me how to sing her music and me telling her how to sing mine," said John. [P] Bonnie, who seemed choked up at several moments during the concert, said she was especially happy to perform with her father now because "he's singing with more depth and emotion than ever." Besides, she noted, "I'm more mature now. [LB]My band and I) don't have to prove how hip we are by doing [only[RB] rock. We can do sentimental ballads." [P] Could it be that show tunes will appear on Bonnie's next album? She shook her head. "It's been done," she replied quickly. At that, John interjected hopefully, "Well, you might throw one song in..." Oh, Dad! [P]