[sophistacavy]

It is very likely that the wildlife defecate all over your yard, but if you can find a spot where there doesn't seem to be any poop, then you could try that maybe after you rinse it off or something. I would keep a very close eye on them, and don't let them eat the grass. Or, you could buy a tarp, and make the pen on top of the tarp so they don't have access to the grass at all.

So basically, I wouldn't let them eat the grass, which would be a very hard thing to do, so you should buy a tarp and put that under the pen instead, for safety's sake.