I do not know if it matters but 10 mph on a 40 MPH pitch is a lot different then 10 MPH on a 60 MPH pitch.
As she picks up speed on your fastball her change up speed need to pick up too.
DD uses a flip change, if I had to start over I think I would have taught her a different CU that follows in speed with her fastball esier.
Out of curiosity, why would you have had her throw a different changeup? Reason I ask is because my daughter who is 11 throws the flip and she kills with it. Her cruise speed is around 49 now and she throws her change at around 37. It really works well and I haven't seen a better change up (or really anything close to as effective) in her age group in the tournaments she's played in. Your comment concerns me because she's committed to it and because it's working so well I'm worried she'll need to change it at some point?