I like the charm packs for kids to sew. They even make a sudoko quilt kit.
I would suggest having the fabric cut before hand so all they have to do is sew.
Yes, I plan to already have the squares cut, so that they can arrange them, in the order they want. Just got back from WalMart, with a bunch of pretty fabrics. Still trying to decide on the size of blocks and finished quilts.
Put a pile of post it notes with a large rubber band or tape them down so the seams are all the same. The kids butt the fabric up against it and the seams are. My 6 yr. old son says put the stack next to the right side (1/4" inch seam) then let her sew at the slow speed, not to fast or she could get hurt.
Great idea! Thanks to you and your son! I'm a little wary of letting the 6 yo girls use the sewing machine but I'll definitely try this, with my 9 yo granddaughter. For the younger two, I'll have to play it by ear. Depending upon how much time we have, I may be able to sit with them individually, to control the speed and watch their fingers. 
He likes the cotton batting- spray baste and let them quilt in straight lines.
Nessie: Have fun.