I don't see any problem with using poly batting with a cotton top - we do it all the time with new quilts. I use mostly cotton because I have a lot of it on hand, but either works well. I would instruct the daughters in care of the quilt, and try to make it as stable as possible. Hand quilting will not hold up as well as machine stitching, but it is more in keeping with the original piece. I have rescued a few vintage tops and machine quilted them on the longarm. The closer stitching keeps the fraying edges in check. Since you're hand quilting I would definitely replace any blocks that are damaged.