I do a garbage bag taped, than a blanket on top of the garbage bag to reduce noise and then the sheet over it all. This way if there is an accident, I can just wash the sheet and blanket and hopefully the playpen mat is safe!

Once they are potty trained though I don't like to have them in the playpen for much longer. I don't feel that it's fair as they can't get out if they need to use the washroom. I transfer them to a sleep mat and watch them on the video monitor until I am confident that they will stay there and not play around. I am looking into buying 2 cots though so that I have a choice of mat or cot for the potty trained and older children.