I'm not the most qualified here to give pitching advice but I'll take a stab anyway (isn't that what message boards are all about?

). What I'm posting is basically what I've learned from watching DD's lessons with her pitching coach. Getting her a good pitching coach would be the best bet for your DD as a good coach has seen all the mistakes newbies make and can correct them while explaining the concepts behind them.
The trick is to make something that initially feels as unnatural as possible to a newbie such as the windmill pitch and relate it to a natural part of the game. Can your DD make a good, fundamentally correct overhand throw? If so, the 12 o'clock to 5 o'clock position in the downward portion of the windmill pitch is largely just an upside down overhand throw with the finish being the snap against the hip which one can't do when throwing overhand. At DD's first lesson with her coach, he had her warm up throwing overhand and then told her to freeze in the finish position. Then he told her to slowly stand upright and bring her throwing hand back to the finish position of a windmill pitch explaining the similarities between overhand throwing and windmill pitching.
I say this because to simply tuck in your elbow at the 4 o'clock position may not help your DD understand the concept as to why she should do it. When throwing overhand it's the throwing shoulder that generates the throw and the elbow is simply along for the ride and to help snap it at the finish. The same thing is true with a windmill pitch, the elbow doesn't generate the mechanic it's role is largely to be the hinge for the snap and release. Try alternating K's with overhand throws when you practice w/DD to reinforce this concept.
Good luck to you and your DD, I hope this helps.
Dusty