Call back. Some people are getting CMS that tell them this when they're just trying to pay to upgrade their dining plan. You should definitely be able to make an alteration like that or like changing a child's age without losing the promotion.
Remember you can't just add a 9 year old to the adult plan - you have to change their age on the reservation to 10.
As for the original question - probably yes, but at some times you may encounter a server who tells you the kid has to order from the adult menu if they have adult credits. There appears to be no rhyme or reason to this, but there have been multiple reports that the Plaza Restaurant, for one, does this. Also ask about larger portions of child's menu meals. Some restaurants may do this (but not all). You can also pay for the kid's meal OOP and use the adult credit another time. If you do this enough, you may have enough credits left to throw in an extra meal for the adults, any kids in the party can get kids meals OOP.