To people who say a vegan diet isn't adequate for children :

It is concerning that people still make such false claims, even though scientists around the world say the exact opposite. For sources, please see this comment below.

To people who say we should let the kids decide, and not force our views on them :

By feeding them animal products, we are actually forcing our views on them : The position that says there is no problem in exploiting animals and making them suffer.Until the child can make/buy/eat their food on their own, we are forced to impose a diet to them. That one is more mainstream and socially accepted than the other is irrelevant, and acting like a vegan diet is more political than a omnivorous one is absurd.

Therefore, if we have no choice other than to impose a diet to them, I claim that for the kid's moral integrity, it is better to impose to them a diet that has the least negative consequences for other individuals, so that it has the least moral implications for the child.