The answer you will probably get over the phone is that they will charge the full charge for whatever ae the child is. (People have posted a number of times that they have done this and always got the same answer).
When you actually get there though, the server has some ability to decrease the charge. People have reported being charged an "Entertainment Fee" as was already mentioned. If you think if the character meal as a character encounter for the child, rather than a meal, it does make sense that they charge at least something for the character part.
Usually, when we go to a character meal, the server has seen that DD ate very little and everything she eats is pretty mcuh from the children's menu, so that's what they charged us (rather than as an adult).