I have exactly the same set up - full size crib and play pen in a room with two toddlers napping. My old kids (3+) have quiet time on mats in the playroom but most don't sleep. There are days they are a bit silly and take longer to go to sleep but for the most part they know it's nap time and they put themselves to sleep with little to no help from me. I do also have black out blinds in the room they sleep in which I think helps. If you have the option/availability to add these I'd recommend it.