Nine is a great age to start quilting. The first thing I'd do is ask the mama if she could come to YOU (not necessarily your son's day) more often to work on sewing. Maybe stay for a weekend once a month (although at that age they really like to get things done). Of course you'll spend a lot of time talking/showing about fingers and needles (I'd get her a purple thang if you don't already have one)! Try to let her go through your stash (or the part of your stash you are willing to share) w/o too many suggestions from you. Let her fly on her own; if she decides later that she doesn't like some aspect of "her" quilt, she can do it differently next time.
Eight to about 12 is a wonderful age to begin learning to sew/guilt. For both boys and girls. They aren't so socially conscious yet and more willing to try things, and yet have a sense of "working". Younger and older are fun also, but by the time they get into their teens time becomes an element.