We're having ours on December 1st. It's a Saturday, we're doing it from 10a-1p. We'll have games, show cartoons, and have a pizza lunch. We're limiting it to 100 kids and they have to be kids that go to our school. We're charging $5, and we'll have between 10-20 helpers.
Since you will be responsible for other people's kids (as a babysitter-type service) I would check with the school. They know how many students they can have per classroom. Depending on the age of the students, there are different criteria. So registration will have to include tracking the grade of the student.
You need plenty of parents on hand to avoid any possibility of someone getting hurt, sick, etc. or the PTO is liable.
Have plenty of activities for them to do - and a large variety as well.
Keep everyone in a small area of the school utilizing only a few places (cafeteria, gym, art room, classroom) for various activities.
I don't know of any legal rules. The closest I can come to thinking of legal rules is daycare staffing rules that states sometimes have.
Setting this aside, temporarily, though, there are practical issues to consider. What are you really comfortable with? What ages are in your group. How many authority figures will you have? My thoughts are any adult that doesn't mind standing up and saying you're not going to do that can be an authority figure in elementary school to younger children, but by the time they reach middle school, you need teachers and administrators.
The more you have plans to keep the students engaged and busy, the less trouble they are likely to get into. We did a movie night last year in December. This year, we have a movie night in September and decided that
the students would rather talk and due to the fact we had to have a microphone next to the sound output of a speaker, and people couldn't hear the movie, we needed to not do another movie night for a while.
Your facilities dictate how many people to have to have to guard the doors and halls to the restroom.
Do you have an afterschool group at your school? Perhaps they can give you ideas.
I must be crazy, but I'm planning on hosting a Parent's Night Out in December. Does anyone know any legal rules for hosting a parent's night out. Like the # of adults vs. kids, emergency contact documentation, etc. Or where can I look?