Yep, mistake. You can put them all in one room and expect them to be QUIET and not play. It will require some re-training on your part and you will have to sit in the room or near the room to put a stop to any nonsense for a bit until they learn that quiet time is QUIET time.

I have five kids between 15 mths and 3 years old and they ALL sleep in the same room for the duration of naptime with NO fooling around.