When DD1 was the same age doing the same thing, I put her in a sleeping bag/Grobag. I was more worried as it was coming into winter and she kept coming out from under her covers. The bag meant that she couldn't get the leverage to roll over for at least a few more weeks (I think). By the time she figured out how to roll in the bag, I figured that she was probably old enough and mature enough to find her most confortable sleeping position, and left her. Plus, the bag was just like a blanket, so she was still warm.