What a wonderful grandma you are!!!
Tartan has provided a great solution. For storage that long, you need to roll them or keep them flat. For rolling, you can also use a length of PVC pipe from the hardware store. The quilts should not be in direct contact with wood, plastic or cardboard, so if you do roll them, you need to cover the tube first. Plastic containers are NOT a good idea at all, the quilts need to breathe and the plastic can trap moisture and ruin them. If either of these methods don't work, then you need to fold them on the bias to minimize permanent creases and store them in a pillowcase on a shelf out of sunlight. No plastic tubs! They need to be taken out & refolded at least once a year. There are several methods you can find on line for diagonal folding, here is one:
http://appliqueandpatches.blogspot.c...t-folding.html