Do you let your baby sleep on a bigger mattress (instead of cots and playpens) just so that he can learn to flip? I have a 4 month old who may find the playpen too small for him to flip or once he learns to flip, he wont be able to continue sleeping in the playpen. So im considering to buy a maattress to put on the floor. It is such a waste of the playpen. Do other mummies have the same concern?