"The concept of treat food just buys into the narrative there is good food and bad food."
I agree 100%, @JacquesHammer. I don't think any food is bad - or shit - but there are some foods that should only be eaten in moderation, and occasionally. Learning that is vital - and in my opinion, it would help combat obesity.
I also think that, if you restrict a child's access to certain food groups - biscuits, sweets, fast food, then when they can access those things freely (when they have pocket money and are walking past the shops on the way to school, for example, or when they go off to university), there is a big risk that they will go overboard on these things - it is exactly what I did, because I had never learned to self-regulate.
Obviously, when a child is of playgroup age, they can't be expected to self regulate, but even at this young age, the odd sweet or biscuit will not set them up for a lifetime of obesity - as long as it is within a balanced diet.
If I were @neonyellowshoes, I would not be withdrawing my child from the group - because the social interaction is a good thing - but I might be suggesting that the snack should be fruit most of the week, with a biscuit just once a week, perhaps - and suggesting milk instead of juice or squash. Or I'd be taking a snack and drink for my child myself - and making it nicer than the biscuit - which doesn't have to mean unhealthy. A pot of strawberries or melon or pineapple would tempt me far more than a plain digestive, and I am sure the OP knows what healthy food her toddler loves, and would prefer to a biscuit - but which would still fit in with her philosophy about feeding him.