I'll clarify. If you are
DDP, then kids meals even at signature places are inclusive. At regular TS places they are inclusive: beverage and two sides. A side can be a veggie, dessert, fruit, etc. It kind of depends on the child. It' also depends how picky they are. Like some kdis might not eat the veggie with some meals. I woudl say they are similar to most kids' meals, perhaps slightly bigger than many.
I would also say the CS places are about the same size. At many CS places they get a choice of entree - like a burger, pasta, pizza- then they also get two sides. The standard sides choices are things like :grapes, apple slices, jello cup, applesauce, french fries, pudding cup. The grapes used to be a good sized portion,but now they are kind of tiny - I think they are about the same size as the bag of apple slices that comes with a happy meal. the pudding and applesauce and jello are all the standard 'snack' size. Mac and cheese is a common choice- it's very basic Kraft-like Mac and cheese. (once upon a time each place made thir own from scratch).
At any buffet or family style place, the kids get as much as they want of anything.
One thing about ordering off the adult's menu for kids is that the choices are geared towards adult's preferences, but - for lack of a better expression - the tastes of Middle America. You won't find much that's very spicey or truly exotic. Most TS places offer a basic steak - even though the description may sound more exotic.
Still one more option might be to get an extra appetizer. You can see all the WDW menus online, if you want. Personally we find however, that even twith allthe walking - there is more than enough food in adult entrees to share. We frequently share entrees among two people. Some fothe QS places are, IMO - especially huge portions. And often - at QS palces - you can get an idea of the sizes simply by looking around when you arrive.