I did quilts for my DD's class project for the annual auction. One woman always questioned my quantities. What was her expertise? Her FIL was in the schmata (that's Yiddish for "rags") business. I offered to let her take over. No, she couldn't because...guess what...she couldn't quilt. She is also the woman who saw the kids' handprints for the back and told me (sight unseen) the quilt was not colorful enough. So, stick to your guns. You are making this a project where teh kids have success. Not frustration. Not failure. Do what you know. If there is extra, which will be a half-width or so of each piece, make a charity quilt for whatever charity the kids choose. You'll have enough for a front and back that way. A king quilt takes about 10-11 yards. NO less. if the quilt is 108x108, that is three yards wide by three yards long. You do not want your backing shorter than the quilt. You want to have some extra room. One yard split three ways (across the three strips) is only 12", that's 6" on top and 6" on bottom and 6" on each side and more work that way too.