Honestly, I wouldn't worry about it too much. I found it really hard to put Katie a certain way once she realized that she could change it. Funny, she actually spent way more time on her tummy after she started rolling, but I think your LO is probably just fascinated with the fact that she can switch back and forth. If you want, you can put some toys just out of her reach so she has to roll to get them....but I still wouldn't expect her to stay long on her tummy. If you really feel she needs to be on her tummy, you could always prop her up on something like a Boppy or a rolled towel so she finds it harder to roll. I wouldn't think it would be necessary though.
Hope this helps!
