@Alx7 you know your DC better than anyone else and if you see them react to sugar, they react to sugar.
DD7 reacts to sugar, not emotionally but physically - aside from being more tired and sluggish, in particular her PH balance messes up and she ends up with a sore vulva (like thrush or start if a water infection).
It is also linked with how much water she drinks but we know that pattern and how much sweet things she eats at MIL’s and she does too now she’s older.
As others have said, everything in moderation. It’s DC’s choice how many sugary foods they choose from a birthday buffet table but it is a parents‘ responsibility to teach children about healthy choices, control and moderation, and importantly also about exercise.
NB. I know I’m judged when we go for a meal out, perhaps twice a month, as I allow the DC a drink of their choice eg Diet Coke (not full fat), lemonade, juice etc. I can see the judgement from serving staff. What don’t people know is that we don’t even buy or have those drinks in our house, they drink water all of the time! It really pisses me off being judged when I know it is a treat and not a common occurrence.