I keep the children in the playpens until a new baby starts and I need to move them on, no matter their age. Then I move them onto cots and there is always a transiton period to teach them that they are expected to stay on the cot the same as they stayed in the playpens.

We all have different setups and mine includes all the children sleeping in the same room, so I sit with them until they are asleep, then I can go to a quiet room and read for my break. Then I'm back in the room before they wake up so I never run into problems. But it sounds like you might have different sleep rooms so I'm no help!