I agree, She still needs to work on taking some speed off the changeup. She has been begging me to learn a new pitch. I haven't gave in as of yet. Her pitching coach is suggesting that she learn a screwball. I have read mixed reviews. He thinks she is ready to move forward. I am not sold on the idea. But I am out numbered!!
There is no magic one size all progression. Every pitcher is different. Assuming she has adequate control of both the FB/CU suggest she play with a few pitches and find the one that she is most naturally inclined to throw. Then move to master that so long as it is not at the expense of other pitches. It may be a crew, it could be something else. At 14 years old I worked with my DD to learn a traditional curve. She ended up with a rise and has never been able to master a traditional palm up curve.
Change up. Current pitching coach has always said learn to locate inside and outside then we will work on high and low. Basically he meant once your DD can locate fastballs and changeups on each corner, then he teaches the drop and rise.
Anyway I agree with the posters above. If adding a "3rd pitch" keeps her from getting bored go for it but at 10U location of the FB and CU are king and will serve her well as long as she pitches.