Is your daughter a happy, healthy child? If she is, then you are doing an excellent job in bringing her up.
Some people are born thin and skinny and they will remain so for the rest of their lives. They will always look younger than they are, while the rest of us will always want to be as thin and good-looking as they are.
Sadly such thin people, even when they are healthy and happy, often believe that they have a problem with gaining weight, when in fact they have no problems - they are just happy and healthy as they are.
When people ask you why your daughter is so thin, tell them she is a very special child and that you are proud and honored to be her mother. And please ensure that you always tell your daughter how beautiful she is - there will be enough insensitive people who will want her to believe the opposite.