Personally I would leave her be. All of my kids have been tummy sleepers as babies. Isla is about 2 weeks younger then your girls & she is on her tummy too. The way I see it is if they can get them self on to their tummy then they are pretty right. Just make sure there is nothing in the cot for her to get up next too & that she is on a nice flat/firm matteress with the bottom sheet tightly tucked in. Isla has been sleeping on her tummy for over a month now.