Maybe it is a "buffet" thing.
If you are eating a child's meal off of a child's menu, then you are definitely eating a child's portion - so it makes sense that you need only pay for a child's portion.
If you are eating at a buffet, what is there to stop you paying for a child's portion, but eating enough for the average adult? Obviously some 6 year olds probably eat a lot more than many 12/13 year olds - so the logic is flawed. But thet need to draw (and enforce) the line somewhere. So I would imagine that they tend to be stricter at buffets.
If that is sensible logic

Alice