my take-away when ds was younger (and not touching the curve for *height*, so might be different than being low percentile on the *weight* curve, so total grain of salt needed here) was that before age 3, growth (again, heightwise) was due to caloric intake and not hormones so before that age small kids need all the quality calories they're willing to take in. if she likes pediasure, fine. if not, the other suggestions all seem great. now, if she's just petite & so is the rest of the family that's certainly less of a worry!
and ds is starting to catch up some in the last year or so! just below 10th%! way better than 'hey, where is the line?'