I taught my 11 year old GS to piece one summer and he continued to work on his quilt at Christmas with me. I've had kids as young as 8 in classes; often they are better students than the adults!
All the suggestions you've received here are really on the nose. You will have the treasure of a lifetime when you spend this time with your grands! Just wish I had more of them to share the experience with myself.
Jan in VA