Thanks for your question. Most pitching coaches will emphasize speed because they are trying to get the mechanics of the wind mill pitch (snap, push off, hip rotation) down first and then will hone control second. 42 MPH at 11 is pretty impressive. I understand what you are saying about control, because it doesn't matter how fast it is if you cannot throw a strike. Nonetheless, I would trust the pitching coach. He/she will next focus on the release point of the pitch (with the snap) which will bring the pitch under control. Here are a few drills focused on control. This repetition will enable your granddaughter to be more consistent with her release point.
Next pitch up to learn should be a change-up. Perfect complement to keep the hitter off balance. Hitters will eventually catch up to a fastball. The key is keeping them off balance and guessing.