My experience teaching is that kids 8-10 years old are happier with the 15# bow. Of course, they all have different draw lengths, and she may well not be pulling 20# yet. Or she may be stronger than most of the kids I teach. If you get it for her, don't get rid of the 15# bow until you're sure she's ready for the 20# one. If you're getting it for her for Christmas, make sure she understands that you're just as happy if she keeps shooting the 15# bow until she grows into it. If you get lightweight arrows for her, like 1516's, a 15# bow can be pretty snappy.