I agree - the amount of food he took, especially since you said he did this at the beginning of the meal, is pretty rude.
I gotta wonder, though, how many kids were there? If all the kids had eaten only the kids' food and none of the adults', would it have been enough (even without Mr. BigAppetite?)
I'm another one who really doesn't believe in having "adults only" and "Kids only" foods. That's not to say that I'm averse to having "kid friendly" foods. But if so, I'd make as much as I would of any other dish. In other words, there should be enough of all dishes that if everyone took a reasonable amount on the first go-around, there would be more than enough.