That's when you point blank tell the parent that they will be receiving an invoice for carpet cleaning, upholstery cleaning, etc. every single time that their child has a toileting accident. Then when they start to protest, hand them your policy with the highlighted section that states "All children are required to wear pull ups until your are confident that they are showing initiation to go on their own!"
That usually puts an end to that conversation.
Because we all know that little "Johnny" is so advanced that he's mastered potty training in a mere few days. Watch out next week he will be reciting the periodic Table of the Elements!