I second shopping in the "little kids" section and getting ones with drawstrings. I have a skinny 8 year old boy and we have finally moved up in the 'big boys' section - but we buy XS (4/5) or possibly S (6/7) with drawstrings, depending on how they run. Note, however, that boys' stuff runs considerably larger in the waist than girls stuff. I buy ds' sweatpants in the girls section (or DID until they changed the cut to flare out and the bottom and made them look too 'girly' for him
: (his words, not mine).)
Children's Place had some nice ones (cotton blend, white stripe up the side) in XS with drawstrings that I bought at the end of the summer for ds. I bet these would work for your dd if you can find them.
I also find that Kohl's brand stuff runs small. Target brand stuff runs large. Old Navy/Gap run large.