Unless it states on the menu that there is an upcharge for adults, there shouldn't have been one.

For everyone else, there are a variety of reasons adults would need (noticed the word "need" not "want") to order from the childrens menu. Anyone who's had stomach surgery for one. I have a relative who can't eat much more than a child portion.
And since it is a childs portion the cost should be relative to the portion and shouldn't matter the age of the human consuming it.