all the children here except one are in play pens......last year I started a 22 month old child that could climb out of the play pen. so what I did was listen at the door and everytime he climbed out I opened the door picked him up and with my mean face I said "no climbing out" I plopped him back in and left. I kept repeating till he got it.....even if nap time was over I still put him back in and shut the door for five minutes then went back in and lifted him out ......he needed to learn he needed to stay there till I said he could come out. it took about two weeks ...but the key is don't not smile ....and no other words. good luck