The adult and kid-friendly food sounds kind of like something my family often do
In my paternal extended family, there are a fair few adults aged 40+, plus about 6 people in the 16-early20's age range (dependent on who is there and if anyone's brought their partner), and a number of younger kids.
Whichever family member is hosting generally has kid-friendly options. Chicken nuggets, sausage rolls, ham, mini chocolate bars, etc. If there is a potato salad in the "adult" section, say, then there are plain potatoes too. We're not restricted to ONLY eating from one part of the table - you're allowed to take some of whatever you like. It's just generally the case that the younger kids prefer the "kid" food, so you know if you do provide it, they'll eat.
Now, me and a couple of other people are known to like fairly bland food, so we do eat some of the "kid-friendly" food. I don't take so much notice of what my relatives eat, but I do generally encourage the kids to go before me, and i certainly don't take huge amounts at a time. I'm not going to bowl little kids out the way before they've even picked up a paper plate, just because i fancy eating chicken nuggets or whatever. There's always loads of bread, anyway.
As for the niece's boyfriend in this thread?
I don't think eating "kid food" was intrinsically wrong - I do it myself. In my family it's perfectly fine to take some of whatever you like from whichever part of the buffet. But I do agree with other posters, that it was the amount he took from the kid-friendly food, that didn't leave enough for the kids, that made his actions rude.
At our family gatherings, "kid friendly" food like that is to ensure that the kids do eat. So you can say, "Look Billy, chicken nuggets/sausage rolls/sausages/smiley face potatoes! You like these." Rather than trying to persuade a four-year-old that they should be eating pasta salad or something.