I am going to give you the benefit of the doubt here, and not assume you are trolling with this outrageous story.
Unless this child has severe special needs, there is NO EXCUSE for his behavior. You are not a toilet, OP.
If the parents accept and/or excuse this behavior, then they can deal with the consequences when you quit.
Find a new position, and give only the notice you are contractually obligated to give. If you haven't got a contract, tell them at the end of the day on a Friday once your check has cleared that you are DONE and will never be back.
And if you find a new job and they want you to start ASAP, screw the contract, and walk away.
If anyone ever questions your decision to leave with little or no notice, say this:
"My older charge started deliberately urinating on me when he was X years old. His parents would not allow me to discipline him in any way when he did this or other unacceptable behaviors. Therefore, I left that position ASAP."