One air conditioner uses about 1.5 kilowatts when it is running. The refrigerator uses about the same. So does the water heater, but it doesn't need to operate much.
You can probably guess better than I can about how much your A/C needs to run where your kids are, but over a 24 hour period it probably won't run more than 25% of the time. So it would use 1.5 kw over 6 hours, or 9 kilowatt-hours of electricity per day. The refrigerator would probably use less. My best guess would be around 15 kwh per day total.
To give a comparison, last year we stayed at a private campground in the Michigan upper penisula. The owner had a separate $3 charge if you used an A/C or electric heater. He said he charges separately because their rates are quite high. He pays $0.22 per kwh. I ageed with his logic. That way he didn't have to roll the high cost of electricity into every camper's fee. Since he figured $3 at $0.22 per kwh was enough, I think 15 kwh per day for a round number would be a good starting point for your situation, and that may be high.