If you charge for kids to stay at your martial arts school for a Parents Night Out event, you may be in violation of your state code.
The issue is that you are charging to take care of the children. That makes you a daycare center (even though it’s at night, the law still applies).
The way to stay safe is to make the event free but charge for food and drink. Give the kids a wristband that shows they paid.
Jim King in Orlando charges $30 per child. He averages $2,200 and enrolls after-school students from the current student guest list.