Maybe you can tell your mom directly that it's hard to get the kids to leave so when you come over there could she please not offer them sweets, encourage them to stay, etc. She may need a direct answer. Since she didn't get a direct answer she asked your ds. At least, that's how I interpreted this.
My MIL watches her other grandchildren every day. She picks them up from school and they stay til Mom picks them up. She makes sure they do their homework and gives them snacks. She watches them all summer, too. Personally, from an outside perspective, I do think she's taken advantage of. I think she stocks her house full of snacks and such. She spends her gas money waiting in car pick up lines daily. So just be mindful that even if your MIL agreed to do it she may deserve some compensation.