J.K.
I have 4 children and my oldest (who is now 14) was always in the 10% for weight or less. I worried about the same exact thing when he was younger. He was always tiny. Now he is just a typical teenager. But my other 3 were much bigger. Even my other daughter who is tiny too was in the 25% and my youngest son was 98%. Nothing was ever wrong with my son, just small... Good luck!