I think the age cutoff for a child dining plan vs adult dining plan is 9, same as for tickets, if I am not mistaken. So, unless you lie about her age when booking the dining plan, you will be paying for her as an adult on the dining plan. Now I could be wrong, but if you already prepay her as an adult, I suppose Disney would not have any problem with her ordering off the children's menu, since she would then be getting alot less than her money's worth.
*Edit*
On second thought, even though Disney would surely not object to you buying a child's entree at an adult price, it could be that this would be impossible for technical reasons. Or perhaps not impossible, but very problematic for you. Your dining plan info is encoded on your key to the world card, which would include how many adult dining plan credits and how many child dining credits you have. If, for example, you have three adults and two children eating, but one adult eats off the kids menu, the computer may count all the childrens' menu food as a child's dining plan credit, rather than deduct the adult credits which belong to your daughter. So what could end up happening is that it would throw your dining credit balance out of whack. Instead of using up one of your daughter's adult credits, it might end up using an extra child's credit, which would result in "stealing" a child's credit away from one of your other children. Let me illustrate it this way just for simplicity's sake. Let's say you have you, your spouse, a 12 year old (who counts as an adult if you're honest) and an 8 year old, for 3 nights. So you have 3 adult credits X 3 nights, for a total of 9 adult credits. You have 3 child's credits for your 8 year old. If your 12 year old doesn't like what's on the adult menu and orders off the child's menu, the computer system might automatically deduct 2 children's credits off of your account (one for your 8 year old and one for the food which your 12 year old ordered off of the kids menu), leaving your 8 year old short one meal credit and leaving you with an extra adult credit. And depending on how many nights you stay, and how many times you do this, you could end up using up all of the kids credits out from under the children, and leaving extra adult credits. Does that make sense?
Of course, I use the word "might" because I don't know this for sure. But it seems logical that if the wait staff keys in a child's menu meal, then the system would deduct off a child's credit.
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