Charge a set flat rate per day. I don't do 1/2 day rates, as you can't fill the spot the rest of the time. Ask him to drop off earlier, so there is a consistent routine in place for their day.
I do snack at 915am/lunch 12pm/snack 315pm
Do these kids not nap at your house? Why don't you suggest a 9-3ish schedule, then the snacks and meals will be spaced more appropriately.