Just get the bigger one, then tuck the un-used portion underneath her for a bit of extra padding. She'll grow into it FAST and you won't have to spend for another one
We've got down mummy bags for our two kids. Not cheap, but warm and light. The small one is for kids up to about 40" tall. Rated 0degC or 32F
http://www.mec.ca/product/5025-512/mec-explorer-down-sleeping-bag-0c-kids/