Also keep in mind that there may be more flexibility than you realize in ordering for an "adult child" (that is, someone 10 and over on the adult plan). Are there "basic" items on the adult menu that your child does eat -- things like chicken, or roasted meats, or pasta? You can almost always ask to have something prepared without the sauces, or with different sides, as long as the basic items are on the menu. You can ask for a piece of plain grilled chicken with a side of plain pasta if those items appear within other entrees on the menu, for example. Also, at the resorts especially keep in mind that all the kitchens are connected, so if you're eating at a more "upscale" restaurant that doesn't have fries on the menu, you can probably still get them because the kitchen is connected to the "family" restaurant that's also in the resort.
We just returned from 9 days on DxDDP with our "10-year-old adult" and had no problem having him eat well, and happily, off of adult menus by talking with the CMs and finding ways to put together meals.