My DD is four and admittedly I spent the first half of her life being a bit precious about food but I have really relaxed, though I have noticed an extreme change in her when she has things full of sugar like chocolate or a bun or ice cream etc. Rice cakes, crisps, fruit, yogurt are all fine even though they also contain sugar.
Within about twenty minutes she gets so cranky and emotional it's unbelievable! My dad minds her one day a week after school for a few hours and I'll often come home to find her asleep on the sofa after a sugar crash and he would say oh she was hard work, it will also turn out they went out for ice cream or had a cream bun etc so I filled the fridge with plenty of snacky stuff (like that mentioned above) and asked him to knock the sugary snacks on the head. The difference is obvious, she's not falling asleep and they're having a great time! Every birthday party we go to ends in tears as I let her scoff away with the other children, I had to start bringing our own snacks to people's houses as her behaviour after biscuits/bars is just so unpleasant.
I have lots of friends with children who can enjoy an ice cream or chocolate bar without these effects, I wonder is it because I limited these sort of snacks for the first 2+years? Or could it be a medical thing, when she falls asleep afterwards she'll sweat buckets and be soaked through!