at 8 months she should certainly be strong enough to move her head so she doesn't suffocate into the mattress.... from about 4 months my dd rolled onto her belly to sleep, there really isn't anything you can do to stop them besides those sleep positioner things, but i think if she wants to sleep on her tummy let her.... she's obviously found her own comfy position she wants to sleep in. once they are mobile there isn't much you can do to stop them doing what they want in their cot...