I only encountered one place that seemed to not want to let my 11yo order off the kids' menu....and that was The Plaza. We got around it by ordering a kids' meal for our 21mo, which we don't usually do. We ended up playing musical plates....LOL.
I do expect that any restaurant that won't allow your older kids to order a kids meal will at least offer adult portions of the kids items at a higher cost. They offered us this option at Akershus for my 11yo for whom we were paying adult prices, though he did choose an adult item.
You could ask for that and just order one meal for them to share if they're very light eaters. So far I have not seen any Disney restaurant charge a sharing fee and they seem to be very accomodating about sharing.
As long as you're not asking for more food than you're paying for I really don't see why they'd insist anyone order off the adult menu, but then again, most kids' menus say "for our guests age 9 and under". Its not like most of the kids' meals are a bargain or anything.