At TS (not buffet), you can choose to pay OOP or to share--totally your choice. It might depend on what you order--you might want to pay OOP to get a more "kid friendly" choice. However, you will have plenty of food on the plan to share.
You also might choose to buy a separate beverage for your child (many kid meals have beverage included...but if you don't buy a kid meal...). You also might want to consider bringing some boxed drinks of her favorite variety or using some of your snack credits to get juices from the resort marketplace.
I can promise you that we had WAY more food than we needed or wanted, and there was plenty to share.
Some other sharing tricks--if you get a CS cheeseburger meal, you can order the double hamburger/cheeseburger and purchase an extra bun for 90 cents. Since these are 1/4 pound burgers (1/2 pound in a double), you have plenty to share.
Breakfast is not a terribly good financial use of CS credits. If you will need to buy anything OOP, I'd buy the breakfast. However, you can use a snack credit and get a whole box of donuts. I really don't think you'll need to buy anything other than a few drinks or maybe a hamburger bun OOP even with sharing!