What is a reasonable bedtime for an under ten-year-old?
I agree that children under the age of ten should have that early of a bedtime. If she still wants you to watch her kids at your house every Friday, then she should respect your house rules. And she should teach her eldest son to respect his elders! In my house, weekdays and weekends do not matter. 8 pm is the latest my children will be able to stay up until at least middle school. Unless the grades and behavior are magnificent, no argument could sway me. Children need their sleep. Tell her and your son that if she does not like your rules to find a new Friday night babysitter or to come home by 8pm to pick the boys up.
