The amount of discipline a parent (limited by the time and willpower a parent can input into the process) can impose is finite. Once we accept this hypothesis, we can see how usual norms are imposed by parental discipline plus non-parental discipline. A parent going against a norm will have to use an extraordinary amount of time and willpower to impose his unconventional norm, as he will lack the enforcement of the non-parent actors. Eating sugary cereal is a societal norm in today's developed countries. Therefore, a parent wishing to diverge from this norm would have to expend an extraordinary effort.

He's also going against the professionals who already succeeded in imposing the norm and simply have to maintain it today.

As a parent I would agree that there is little education. Health professionals (in the UK) don't seem to care. Products you think are safe are then found out to be harmful in other ways.

I don't think education is the silver bullet, but habit forming is. For example, what is the habit you resort to when deprived of sleep, late for work and kids are throwing a tantrum? Sometimes that habit lies in preparation, something most of are are not very good at.

Raising children is hard enough without segments of society actively fighting against you.

Or they are exhausted from working all hours because the banks create credit from thin air to maximize rent extraction and lack the willpower to argue with their children every shopping trip.

I have kids, it's a constant battle even though we have no TV and I'm fortunate enough to have enough time to educate them. I'm constantly fighting with a whole room of people who are disgusting scumbags with access to millions of advertising spend.

Most likely the latter, as well as advertising to children to use them as emotional leverage against the parents.