It could be that he had seen television commercials that show those ads. It seems he may not pay attention to the broadcast television, but often they are aware of what appears on screen. Advertising is very interesting because the action and voice. One of the most common type of ads aired on children's programming is cereal. Some of these cereals are nutritious, but others are not. In fact, some of these cereals contain sugar in an amount more than three pieces of chocolate biscuits or as much as that contained in a donut. They're more like a special sweets than breakfast menu.
How can you know that the cereal contains more sugar? Look at the label found on the packaging composition. Composition sorted by the largest amount of content. If the first or second on the list are sugar composition, sucrose, or corn syrup, then the cereal contains sugar in large numbers. It would be better if you choose other cereals that contain less sugar.
How do you convey to your child when you decide not to buy the cereal? Say, "This is not the food eaten every day, we will buy the cereal that will help you grow healthy and strong." Check the labels on other brands of cereal, and let him choose one of the cereal-cereal sugar content low.
If your child is unhappy because you do not buy the cereal she wanted, go off the shelves of cereal. Go and finish your shopping. You can read the labels of cereal next time when you are shopping alone. Please note the sugar has other names that may confuse you, here's another name for the sugar, corn sweetener, corn syrup with a high content of fructose, dextrose, fructose, glucose, lactose, maltose, molasses, and honey.