Generally speaking, yes, you will have plenty of food.
For breakfast, use your mug for a drink and then use a snack credit for a baked good, fruit, or yogurt.
Each counter service lunch or dinner meal comes with a drink, entree, and dessert.
Each person will then have one more snack for later in the day. There are not "kid snacks" so the snacks are relatively large for a child. In addition, the kids can share snacks.
For breakfast each has a mug of hot coacoa and one gets a fruit bowl while the other gets a huge muffin. They split each and have a nice breakfast.
If you are at the magic kingdom stop off at Sleepy Hollow and get a funnel cake and slush. These are huge, and by splitting they get a food snack and drink in the afternoon.
That only works if they can share together, but the plan also works well between adults or an adult and child.
At Epcot, head to the bakery in France and get a quiche, Napoleon and two waters. Amazing snack or small meal for only two snack credits.
Keep in mind that if you use a meal credit for breakfast, you can just use your mug for the drink and use the "drink credit" to get a drink with a screw top lid to enjoy later in the heat of the day.
When on the QS dining plan, many spots have pre-made plastic wrapped desserts. They are not nearly as tasty as fresher produced items. Sunshine seasons, Columbia harbor house, tangerine cafe, STARRING ROLLS, the bakeries, and the resort QS tend to have better selections than many others. As a general rule, when looking at menus, if you see chocolate cake, carrot cake, and sugar free brownie listed together, they are plastic sealed.