I remember cereal at Topsiders every morning and cereal was also available from room service every morning. I don't remember Cheerios specifically but they did have Froot Loops and KO's.
They have snacks available onboard. I remember Pringles, popcorn, pretzels and peanuts that you could buy at the snack bar. If your toddler only eats a specific type of snack I would bring it aboard. It takes up little space and you won't have to stress trying to find what he likes.
There's so much food on the ship, especially food geared towards kids I wouldn't worry about him being hungry. Around the pools they have pizza, chicken fingers, fries, fruit and ice cream (among other things) available all day. There isn't a lack of food to snack on when your hungry.
