Try not to worry about your childs weight.  If her levels are OK don’t worry about her being a bit underweight.  My youngest  SWCAH was really skinny, petite etc...he was not even on the chart for a while, maybe from 16months - 2 + years.

He is now almost seven and he is still short but up to the 50% for weight.  I am actually now watching what he eats a bit.  Just give it time.  Let her eat at meals and two small snacks.  She will catch up or remain skinny, which is OK.  Also, at the time my son was so skinny,  he was bit oversurpressed,  I thought the same thing,  well he should have a big appetite, but he did not?  My oldest CAHer is slim and tall.  My little one is short??? My boys are 7 and 9 years old.

 

Good luck and try not to worry!