If no school, I would charge normal all day rate per child.

Overnight care not cheap. Sure, children will be sleeping but you are still responsible for the 24 hours, it still impact on family plans, it still mean that you unable to socialize, have supper plans with friends or whatever might normally happen without them. You still have two lot of bedding to launder and change. Extra shower or bath water.

I would charge $100 per child for overnight responsibility and restriction on personal time. Plus standard all day rate if children school not open.