I don't agree with letting them learn the hard way without at least trying to help them learn the easier way first. That could turn into a "why didn't you tell me?" situation.
I agree with the sandwich equation of how to discuss problems with the quilting, and with suggesting they double check the directions on the batting and telling them why that is important! Just saying something like, "I'm concerned that you are going to need more quilting to keep your batting from shifting, but check the batting directions just to be sure it is ok." You will have expressed your concern without being critical, and offered a way for them to check on their work on their own initiative.
Edited to add: I didn't understand that you had already spoken to the girls about the quilting, only to the one's mother. If in fact you have already expressed your concern to the quilt makers themselves, and they were not receptive, then I would have to say to let it go and hope for the best.
Last edited by wildyard; 03-31-2013 at 06:57 AM.