I am not a pitching coach, but have been to a Hillhouse clinic as well as a Doug Gillis clinic and my understanding of a peel drop is this. ?You are correct that the grip, for the most part, is the same as a fastball. ?The difference is that with the peel drop you are trying to create as much frontspin as possible, so as the ball travels through the air, the wind resistance against the laces will force the ball to move downward. ?That is it in a nutshell.
If you talk with "most" men pitching coaches, they say there is no such thing as a fastball, they don't throw a fastball, because they want all their pitches to either be moving up or down. ?So in essence, if your daughter is throwing a fastball, she is already throwing a peel drop, just not creating enough frontspin to force it to drop.
Hope I got it close enough for all you Gurus out there.