I'd try both of the above. If she's weeing and pooing in the toilet instead of a nappy she can't get to it to play with it. Unless of course there's a pile of clothes and she does it there instead of the toilet. I guess just leave clothes in the laundry and shut the door.

Plus if she's using it to paint with, it sounds like she would appreciate crafts she can get her hands into, like cloud dough (playdough made with corn flour and conditioner), thick finger paints, coloured shaving cream etc.

I'm also wondering if the little girl is bothered by the smell at all? Not that I think there's a problem but surely that would turn her off?