I have an 11yo as well, and we just returned last week. For all Disney purposes your 11yo is an adult. Even when you're not using the dining plan (which we weren't, but will in September) there are restaurants that expect a child of 10 or over to order from the adult menu. At buffets and fixed price meals there is no choice but to pay adult rates for a child that age. At Akershus we were told that as an adult he would order from the adult menu, but if there was nothing he liked he could order from the kids' menu and get an adult portion.
The only restaurant with a regular menu that seemed like they'd be insistent upon making him order an adult meal was The Plaza. As my ds prefers kids meal food a lot of times he did order off the kids menu whenever he wanted to. At the Plaza we just ordered enough food to go around and all shared (though technically we got 2 adult sandwiches and 2 kids' meals....ordered for our 5yo and 1yo dd's).
But anyway, specifically to answer your question...on the dining plan if you're paying the adult rate you order adult food, if you're paying the child rate you must order from the kids' menu wherever its available. But a child of 11 is an adult and will not be expected to order from the child's menu...in fact, some places even frown upon it.
