I have noticed that in 12U, especially second year 12U the girls are learning to recognize the change up and is able to keep their weight back and drive the ball. We use more offspeed than change up. But if even you use a regular offspeed and try to paint inside, outside corner the ball is sure to get hit. We use a offspeed screw, movement to inside corner (right handed batter) and use the triple pitch, which is offspeed that curves outside and drops.
My dd knows several change ups, but only uses one, maybe two in a single game. There is no advantage of knowing this many change ups, she would learn one and if it wasn't working she would learn another, we practice all of them so she won't forget the muscle memory of the snap, etc. But a change up is a change up know matter what grip she uses.
We never ever, use the fast ball in a game, only when she pitches to her team mates does she use this pitch. Use the breaking ball high and tight, back the batter deep in the box, then come in with a drop ball that drops right behind the plate, next pitch outside curve where she can't reach the ball or with a inside curve where she is bailing out of the box to a strike on the inside corner.
I would have to say if she was just pitching to a catcher with no batter. She will not be able to pitch 10 perfect curve balls in a row called for strikes. Or for any pitch for that matter, she gets her strikes by making the batter chase. In all reality in a game she is probably pitching alot more balls than strikes. She usually allows one walk a game, maybe two. Last weekend she struck out the first 7 out of 8 batters faced. Went on to strike out 20-23 batters in three games.
What I am saying is that movement is the key. In 12U you have to have more than a fastball and a change up.