Seriously........WHY are you asking this question? NOT being sarcastic or harsh but if a parent will not abide by your contract - ESPECIALLY when it comes to payment - you TERMINATE them and move on.
If you worked out of the home and on salary and had a sick day you were entitled to be paid for and your boss told you they were not gonna pay you for that day would you keep working there??
Honey, no one is picking on you. We are just tossing out some tough love so you will toughen up to and see that what you do has VALUE and you are worthy of respectful parents who don't think they are in charge of YOUR business. Tell them the contract is NOT negotiable.
Seriously, tell them straight up that they pay or they are gone. Period.