I have only had this happen a couple times. I have the kids sleeping separately, right now anyway. I will ask mom/dad if they want me to go get the child or if they would like to go get a coffee and come back when naptime is done. So far they have all said, "oh, we'll take him/her now". It's never been right in the middle of nap, and never bothered the other children. Should it be an issue, no, I would not allow it. I would ask that they come back.