My sister did. She made a sandwiche for my nephew and put it in her bag. Usually she'd order him some fries and a drink, maybe dessert if he ate his sandwich. My nephew is extremely picky with his food.
We had to do this a lot when my oldest son was younger. He's autistic, and didn't eat much. Thank goodness he's gotten better about it. We used to bring pbj etc. in a bag, but be sure to order his drink and possibly a side and dessert.
I wouldn't do this at a buffet, though. Most of the buffet style dinners we went to last time even offered stuff to make your own pbj.
If you check out Allearsnet.com, you can find the menus for every restaurant on Disney property. That really helps with my planning. That way, you know what you have to work with. Disney is wonderful with accommodating special requests, so you may not even need to bring your own food.
We have ordered stuff like PB&J,macaroni, nuggets, hot dogs and burgers at several different upscale restraunts. Like a PP said, they cater to kids at WDW. You wont have a problem ordering off menu for the DK's But I wouldn't bring your own food...LOL
The plain roll, yes. Would your child eat something like fries? Or another side? You can just order those things. It doesn't have to be a full blown entree exactly as it's listed on the menu. Plain pasta? If any kind of pasta is on the menu, you can get it plain for your child (or anyone).
I'm just giving you more options. But honestly, if your child sits there and quietly eats his own PPJ sandwich, who is really going to care? Now, if you took in a Happy Meal or a bucket of chicken, that would be a problem.