The kids' menus at Disney are pretty decent.
Yeah. Depending on the restaurant, you can get a smaller sized adult meal for the child. The Japanese restaurant in Epcot is one of our faves for this reason.
My son has asked me to upgrade him to an adult plan 'cause he doesn't particularly like chicken nuggets and cheesesburgers. (common options even at Disney) So next time we visit, I will definitely be paying to upgrade his options.
Sharing meals can be tricky, at least it has been for us. One, my son doesn't like to feel like he's an afterthought. He wants his own meal. Secondly, even when you ask for an extra plate, they often forget to bring one out with the meal and while you wait, the meal gets cold. Third, there have been times when my son wants more and after we share it, it's gone. So sharing for us hasn't worked out as well as just ordering him his own adult meal. Leftovers are great too! He gets to enjoy the meal for lunch the next day.