I have made many kid size quilts with cotton top, cotton batting and flannel backing. I found that pressing with lots of steam helps to keep everything from shifting. It flattens out, but then fluffs up when washed. I had a devil of a time with poly batting though, especially since you can't press or steam without it melting. You might also try to stabelize the sandwich by using dissolvable thread or 505 spray. I spray the sandwich and then pin the corners. That and the pressing is I think what helps me. Maybe some of that will help you.