I am a stay a home mom with 3 kids ages, 5, 6 and 9.
I would tell her out right you took them with the knowledge the boys would be picked up at a certain time. If they leave later than that agreed time tell her she owes you xx amount of dollars per child per hour they are left after the agreed upon time. As for the behavior maybe find after school places to go like a park or craft time inside. Also explain the boys your house rules and if they don't follow them then they get a time out. If parent dosen't like the time out tell her of the child's behavior. You are doing a favor by watching the kids. My friend does day care and she even gives them a time out if they don't follow the rules.
Ugh I get this one too!! I only have 1 DD & she's in 3rd grade but there are a couple of people I have had to tell, sorry I can't raise your kids for you. (For free!) ITA with the other ladies & you need to put your foot down right away or it's going to