I don't understand the questions.
Are you asking, "Is a 10MPH difference between a fastball and a changeup enough of a difference?" If so, then the answer is "Yes".
Are you saying, "If a pitcher's changeup is 40 MPH and her fastball is 50MPH, should she be able to throw the fastball faster?" If so, then the answer is, "I don't know. Ask your DD."
But, anyway:
I believe these are the wrong questions. It sounds like you are fixated on the speed differential.
The key to an effective a change is NOT GIVING THE PITCH AWAY.
Remember that the key to good pitching is striking out the "good" batters. (If your DD is struggling getting the 7-8-9 batters out, her changeup isn't the problem.)
The effective pitchers with a changeup don't give any signs that they are throwing it...either before, during or after the pitch. The batters don't know it is coming.
E.g., if your DD takes more time "getting her grip" for a changeup than a fastball, the batters will pick it up. If she does a slightly different movement of her hands, the batters will pick it up.
At the younger ages, batters are not good at picking up indications of the pitch. As the batters mature, they become better at picking up pitches. So, you can have a kid pitching 14U that strikes batters out with her changeup suddenly starts getting pounded when she starts playing 16U.
Silly,
FB is what it is, everything else is off that.