Ghost of Potty Training Past

As a weak attempt to get Burns #4 and possibly #5 to potty train themselves, I put a mini-potty in the bathroom. Maybe if they see it enough they will want to use it? Instead of the desired effect, I have found myself emptying it multiple times due to the ghost of potty training past. Instead of 5 potty trained kids, I am doing diapers for two kids and emptying a little potty from one of the other three. No one will fess up to being the ghost.