I really like your idea! You are going to be a very popular grandmother. I have taken yarn and instead of sewing it through the quilt, I've sewn in on with my sewing machine...tacked it in place and then tied it. Might be easier and you could use the high loft. A very simple applique pattern could be a heart. The kids could simply draw their heart the way they want and it would be easy to cut out. That just what I would do.