Great idea on the quilt - work with your son on personalizing it. Find a fabric that has her favorite flower, maybe a picture of a pet, school colors, etc.
As far as a pattern - I would suggest the D9P. You can strip piece it together and it is a really great and interesting quilt.
Good thoughts to your DS. This must be a really hard time for him too. Maybe helping you with the quilt will help him with his grief process too. Maybe you can put a couple of pictures of the two of them together on the quilt. Just an idea.