I agree that Quilters Dream Request is definitely the thinnest, lightest cotton batting. As a result, the quilting will have very little definition ... but as long as you're okay with that, it's a good option. Typically, the focus of children's quilt is the color and/or motif -- the quilting is more functional than decorative in that case. Cotton is still plenty warm, though, so I'm not sure I would call it "summer weight", just lightweight. I'm not sure how old your granddaughter is, but you might consider Hobbs washable wool. That would be both lighter weight & cooler than cotton, and it provides great definition for quilting. Wool is not recommended for infants as some have a sensitivity to wool and can develop a rash. An 80/20 cotton-wool blend would also be something you could consider. It isn't a whole lot cooler than 100% cotton, but it does provide better definition for your quilting.