When my mom had her min pin she had to close off the front room from him. We put the couch and a chair along one area and a large chalk board from when we were kids across the doorway to the kitchen. He was small enough that he couldn't jump these obstacles.
We've not had a real tree with Taggart around. Although, my brother does. We shall see how that works out.
I'd say the play pen idea or closing off the room, if possible. But try it out first and see if they are even interested in using it as a potty. Of course keeping a close eye on them. LOL