Tummy time does not prevent a flat head by the way. My son has torticollis which caused a flat spot, regardless of how much tummy time he did when he was awake. He now has a helmet and it's not because of a rock and play. It's because you're told to place your baby on their backs for sleep and if a baby can't turn their head one way due to a muscle issue (even with force by parents) then a flat spot is inevitable.