My 24 month old weighs just over 21 pounds?

My daughter is the same. She was a very light eater and we took her to the Ped many times. Her pediatrician wasn't concerned as long as she was getting liquids in and recommended PediaSure. Even now at 3 years 4 months, she is only 29 pounds. She is active and happy and is learning everything very well, so we aren't concerned anymore. Its stressful, but if her Ped isnt concerned, then just take it as it goes.