I am pretty open to how a pitcher starts their motion but I am firm that it should be the same thing every time.
Every pitcher seems to be different.
My DD starts moving her feet up and down, pitcher 2 starts by putting the ball in her mitt, pitcher 3 starts by dropping her hands, etc.
I am working with 1 particular girl that starts her windup 3 or 4 different ways, drives me nuts. I do not know how she thinks she can get her timing down doing it so many different ways.
Its more of a style issue than anything else. My preference is to simplify the pre-motion as much as possible (glove in front, chest high or at the hip hiding the ball), and as mentioned here, find what feels comfortable for them and stick with it. Make sure that all your pitchers get in the habit of having their hands separated (either ball in glove or ball pitching hand) while on the pitching plate and before starting her pitching motion. This way its becomes second nature to them in the game and won't be called for an illegal pitch. Educate yourself on the rules about "crow hopping" and "leaping" so when you work with these girls they do not get into bad habits.
I would not be too concerned about accuracy right now. IMO, the two most important things for new pitchers are a) proper mechancis and b) an aggressive motion (good stride, fast arm circle, arm whip) to home plate. They should be throwing hard, with a controlled and repeatable motion and will NOT be throwing a lot of strikes in the short term. It's a process that you need to let play itself out.
Lastly, unless you are an experienced pitching coach, you are limited in the amount of time and energy you can put into helping these girls pitch. The parents are the ones that should be working weekly with their DDs and hopefully have the resources to see a pitching coach, regularly. There is only so much you can do, 95% of it will be the pitcher working on her own with her parents/pitching coach. Otherwise you may be setting yourself and the girls up for failure. I frequently have parents ask me to help their daughters become pitchers and I tell them straight up that I if they want to come early or stay late after practice than I can work with them on the bascis but that if their DD truly wants to pitch, they (the parents) need to be the ones to work with them on a regular basis. There is no dabbling in pitching, its too complex, either they commit to it fully or they should choose another position. Just my 2 cents.