Even though we always do the deluxe dining plan, we pay cash for our lunch at Brown Derby when we dine there...which is pretty much every trip.
All DH and I want for lunch is the cobb salad and some wine or beer, so it's not worth 2 credits.
The last 2 trips my kids were less than impressed with the lunch selections and the servers let them order hot dogs off the kids menu even though technically only 1 of them was under 10. in fact, the servers suggested it both times. My older son also orderd a short rib appetizer one time and another time, the crab cake appetizer to go along with the hot dog.
My younger 2 kids got hot dogs and desserts. Since we were paying cash, I don't think the server cared what items they ordered or how we mixed and matched things. Adult appetizer, kids hot dog, kids ice cream dessert for example.
I think as long as you do not have to follow dining plan rules, most servers will let you do what you want.
The only people I have ever talked to that were told that their older kid could not order off the kids menu are my neighbors and it was Flying Fish. Their boys were 12 and 14 and one of them wanted to order off the kids menu and they were told that it's for under 10 yrs old. The CM did offer to alter any adult dish, change sides, remove sauces, etc. They were fine with this, so it was no problem for them.
So, it may happen for sure. I would just be prepared to alter an adult entree to your childs tastes if need be. I'm sure that every restaurant will be happy to do that for you.