But there is a difference between being small for height and weight but GROWING and GAINING weight. The original poster's child isn't growing, isn't gaining weight, and in fact has lost weight. So for those saying "oh, don't worry, your child will just grow slower but will do it" that's not accurate for those whose children aren't eating, and aren't gaining.

My 3yr old dd is 5th% for weight and 50th for height. Both her ped and dev ped say she's fine, just small. I've always been average height but thin, very thin as a child, so like the pp, genetics was an important factor for us.