Just talk to her, it sounds like it was a well intended gesture and I would assume that the money given wasn't intended for a slushee. I feel teachers have it rough and we need to be supportive, they are trying to balance 25 or so kids so it would be unrealistic to expect her to know exactly what goes into your LOs lunch box and how it is being consumed over the day, she would have probably just been aware that at that particular time, he didn't have much food left and appeared hungry, not realising how much he may have eaten earlier in the day

My DS is 5, on the odd occasion he has canteen money, most of the time he makes good food choices, sometimes not. Some days he eats like a horse, other days he's too busy playing to bother.

I think the appropriate thing would be to thank her for her good intentions, but just give her a friendly heads up that you'd prefer she didn't give him money in the future.