I like to keep them in playpens for as long as possible just for safety reasons and peace of mind. Once they get too big, too mature or are potty trained, I switch them to cots or sleep mats. I don't stay in the room either but I have a video monitor that enables me to see what is going on down there. I love it for when I transition children to the cots. If they sit up, get off it etc, I can see and deal with it right away.