Despite a troubled rehearsal period and mixed notices, Once Upon A Mattress has being doing fine, thanks to the star appeal of lead Sarah Jessica Parker and the producers' decision to market the show towards younger and family audiences. The revival of the Marshall Barer/Mary Rodgers musical, which opened Dec. 19, reached its 100th performance March 16 and released a cast recording CD on RCA Victor March 27. Box office figures for the week ending April 6 showed Mattress filling 79% of its seats to the tune of over $300,000. As if to solidify the here to-stay status of the new Mattress, on Sunday, April 13, Carol Burnett -- the original Princess Winifred of 1959 -- caught the matinee and visited with Parker and the rest of the cast. Burnett herself will be honored April 17 by the Library of Congress for her contribution to American Culture; the presentation will be broadcast on the Lifetime Cable Network May 4.

Once Upon a Mattress is going all-out for the kid vote. Taking its cue from the League Of American Theatres & Producers' "Kids Night On Broadway" promotion earlier this winter, the Broadway revival declared Thursdays as "Kids' Nights." Since March 27, for every full-price adult ticket purchased for a Thursday night performance of the musical, the buyer gets one child's ticket free. Patrons can buy tickets either at the box office or by phone at (212) 947-8844.

Once Upon A Mattress opened Dec. 19 at the Broadhurst Theatre, West 44th St.