Depends on what you have... if they are effective pitchers and have PC's, you leave it alone and work on strategy, pitch sequences, etc, etc. I'd forget skill teaching in that case and work on other things. If she can throw a drop ball even if it is not your way, who cares. Call a drop and be done with it and don't try to correct technique in practice or games. If she is struggling in the game, pull her if she can't get it back herself - she should be able to self-correct at that age.
If they suck... well you have all the reason you need (and no real bad consequences) to make changes. What do you have to lose? When your pitchers ERA is 8.0+ and she or her parents complain that you are changing their style you have some real good ammunition to fire back with.
My DD's head coach for travel is a local hitting instructor but only 4 of the team use him The rest go elsewhere. He tells the parent he doesn't care who they go to, he wont mess with their swing if they go somewhere else UNLESS they aren't performing in games. Our normal lead off and 3-5 spot all go to someone else. His students bat 2, 6,8 and every other game. That doesn't appear normal behavior for a coach but it should be,
That's how I was when I was coaching a team. I used to say at least if they went to somebody else I couldn't be blamed if they didn't perform.
I would try to learn what their coaches had told them to try to help as much as I could. Often, though, I'd just ask "What does your pitching/hitting/whatever coach say when you're doing this?" Usually it was enough to get them on track.
There were a couple of times with kids who were just early learning when I'd take a parent aside and suggest they were not spending their money wisely on lessons. For pitchers that usually meant HE and slam the door. I'd tell them you don't have to come to me, but if you want your daughter to succeed you need to find somebody else. And I meant it, although of course they'd come to me. For kids that were deeper into their journey, though, I just let their performance speak for itself.
Ultimately, like any coach, my goal was to win games. To paraphrase the movie 42, I'd let an elephant pitch if I thought it could win me ballgames and I'd bench my own sister to do it. Whether they were my student never really entered the picture. Well, it did one game because of a particularly troublesome parent, but only that once. And I'm sorry it happened then.