I think Mummy Naomi expresses my sentiments perfectly

"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"

I would definitely get her side of the story first. I am sure that the money was not intended for a slushy but perhaps she herself was unable to supervise the purchase from the canteen. Teachers cannot be everywhere at the same time and sometimes have to make quick judgement calls based on what they see and hear at that time.