That's so gross. It has never happened to me, but I do not think I would have the stomach to clean the playpen. If that ever happened to me I would charge the parents for the cost of the playpen or ask for a replacement, toss it on the garbage and and ask them to bring one piece pjs from then forward.
I have a video monitor, as soon as I listen someone moving around or making noises when it's not wake up time yet I ask the offender to keep quiet and they listen.