If the parents are not seeing this as a major issue then I would give them a notice of probation. For them, more so than the child. Then as others have suggested have them supply a lot of onesies (or make it clear you will check each morning and if she's not in a onesie she is not be accepted into care).

Then use the footed zip pjs. On backwards as noted above. The footed ones work just fine...spin the legs so the feet are faced the right way and they still work just fine (my friend had to do this with their 2yr old).

Have the parents supply numerous sets of bedding and send them home to be cleaned (make sure you have enough to use the next day as they likely won't wash over night).

Put as much of the burden on the parents as you can so you are not left with a ton of extra work. They need to work on this at home as well...sounds like the child is doing it there too. Make it clear that with her being in a room with other children she cannot be throwing poop on the other sleeping children as it is a huge health issue therefore they need to be onboard with ending it and preventing it.

It's a stage that even the best raised children can go through...but the parents to need to own up to it and make your job as easy as they can during this stage. The more the parents resist and refuse to help the more responsibility I would throw at them (the laundry to wash, bills for cleaning, new playpen etc). If the parents are all on board with helping and ending it then I would just do everything you can and keep them up to date.