I have a separate room for napping but if I didn't, I would set up a playpen in my computer room, living room or even my own bedroom and use it for the odd times that a child is tired/cranky in the mornings. I would wait until all the children were here (unless there is a big gap between drop off's). I would just quickly run the child upstairs and put in the playpen. I am not a helicopter dcprovider and for the 2 minutes or less it would take me, the other children would be fine. (It's really no different than going to the washroom)

I have blankets on the sides of a couple playpens (so they can't see their friends and for the most part, they know not to touch those blankets) and that would maybe work to make it a bit darker. If you can put a playpen/crib sheet over top off the playpen, it would keep the blankets in place and make it dark. But keep one or two sides blanket free for breathing! If the child is cranky and tired, they will likely fall asleep anyways. I also have a white noise machine and I would put that on so they couldn't hear the other kids playing as much.