I'm not angry at all. I'm not going to go in all guns blazing or anything. I have given DS the money and told DS to give the teacher back and to say thank you. FWIW, this is not the first time this teacher has given my child money. But the first time she did it was for a special day when the kids had a sausage sizzle, and DS lost his money, and was upset about it, so she gave his the $2 to get a sausage sandwich. Another really lovely and kind gesture and again appreciated by myself (and him). In that instance, I didn't mind because it was kind of 'healthy' substantial food, and for a special school day to participate in. And again, I returned the money and thanked her for her kindness.

As I said in my OP, I get that she was being kind and didn't want him to be hungry, I really do appreciate that. But he brought a slushy with the money. That is not going to fill his belly at all, it's just junk food and it's not something I would feed my child. I don't let him buy slushies when I give him canteen money, and I supervise his choices when I do let him eat canteen food, because he's 5 and doesn't know better yet. I just think, if she wants to be generous for her students like that (and it really is lovely that she cares so much about 'her kids') she should take a little more responsibility in it and either tell him what to buy, go with him to make sure he made a decent choice, or get something healthy to give to him herself.

Being an early childhood teacher myself, I know the difficulty first hand in caring for 20 or so children. I know that it can be tedious and hard to monitor that the children have enough food, are eating healthy foods each day, or are not scoffing every morsel in the lunch boxes in one sitting. I've done it. We always had a little stash of healthy, low allergen foods to give to the kids who either didn't have enough foods, or brought in inappropriate foods.The teacher would have to be checking lunches to ensure they're allergy free (egg and nuts are not allowed in their lunch boxes) so surely she would know what sorts of food and how much he generally gets.
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I will talk to the teacher this afternoon. I sent extra food for DS, some more fruit and some rice cakes with cream cheese as extra (unfortunately, he won't eat vegetables with dips or anything like that). His lunch box was overflowing. DD laughed at it, and scoffed at him 'I bet you can't eat that much food'! I told him not to eat it all at once, and to leave at least one or two things for recess.