what to charge for this

2 day school carnival, 8 hours a day. I haven't ever done anything like this. No clue what to charge. What would YOU charge>

Ooh. I do an event each year that's about 18 hours over 3 days, and I give them 25% off. Or do something like 8 hours for the price of...6? Haha, that's the same thing... Or 8 for the price of 7 and you'll knock off travel.

Make sure you write in your contract that you get breaks! I know it's so easy to just keep going, but I think it's better to get breaks approved and not take them, than the other way around.

My day rate is $600 per day so $1200 for the two days, plus travel if necessary. If they asked really nicely, I'd drop it to $1000 plus travel.

But I only paint for 7 hours per day for that rate (over 7 hours is full hourly rate for the overtime!)

They often can get a sponsor, OR, charge for the face painting themselves to recoup the cost.

What Kristal & Shannon said.
Make sure they provide a line manager so all you have to do is paint. Their job will be to keep this kiddos from fighting, help them choose a design before they're in the chair & collect any $ or tickets, etc.
Have a clock that says, when the next break is & when you'll be back. Hydrate! Hydrate! Hydrate! Bring healthy snacks to munch on.

School carnival? I wish I was in the school districts you people live in. There is no way I could get that rate from a school. When I tell them my rates, I never get a bite. I have offered and gotten the gig with PPF instead of hourly.
Let us know how this turns out and good luck!