L.D.
Once they start rolling over you really do not have to worry about the staying on their backs anymore. But there are different positioners you can try.
http://www.walmart.com/catalog/product.do?product_id=2667882
http://www.walmart.com/catalog/product.do?product_id=4631162
http://www.walmart.com/catalog/product.do?product_id=4645524
http://www.walmart.com/catalog/product.do?product_id=2513039
these are three different types