I agree this sounds sanctimonious but...
There is a complete double-standard on MN regarding obese children.
If you had said: AIBU to think it doesn't matter much if my child is a bit 'stocky'?
You would have had everyone queuing to tell you that it does, you are setting them up for a lifetime of obesity, you are contributing to the obesity epidemic, why can't you just eat healthily etc.
If you eat sugar for breakfast (and I agree Weetabix has some sugar, but not more than Honey Loops FGS) and then go and eat a free full of carbs school dinner with lovely pudding to go with it, you are setting yourself up for poor future eating and probable overweight/obesity.
People keep saying 'but I ate that in the 1970's and I'm fine' ignoring the fact that the generation who grew up in the 70's are the fattest ever to have lived!
It may not show on a small child running around lots, but hit mid-life and large portion sizes, mainly carbs and heaps of sugar all take their toll and leave 40/50% of us overweight.
I am not fussed about size but I am about being healthy and being able to move freely, and shitty sugary breakfasts are all part of a wider pattern of being given rubbish processed food for which we are all suffering.
I don't feel better than my friends if I avoid these cereals, and like another poster, we eat them on weekends/holidays/if given them, so no food is taboo. But how have we sleepwalked into this being a standard breakfast to fuel the next generation? The branded cereals aren't even that cheap!