Well, let say you decide to eat healthy, or you always eat healthy, you are starting a diet to loose some weight so you need to reduce your calories per day.
You are pretty good about what you eat and as most diet, you are not supposed to eat much carbs or none.
There are no pizza or hamburger in a diet (usually) and no ice cream or cookies either, but as you are watching your favorite show, you will see many commercials for all those.
So, you had your diner and you feel perfectly fine, you are not thinking about food at all. But as you are watching TV and you see so many commercial about food, junk food that you are not allow to eat, sugary stuff that are not on your diet, that it makes you think about it.
And this is it! TV commercials exist to create needs, they are repeated over and over to penetrate your brain and control your desire. So may be you will go back to the kitchen and indulge yourself.
For children, it is even worse. Their brain is still developing and is weaker than yours. Like it or not, it is a lot easier for advertisers to create needs in their brain, to trick them in fact.
And as you can see, they do not care at all about their health!
The only thing who interest advertisers is your money :-)
And the only way to protect your child from them, beside turning off the TV, is education, media literacy education, meaning teaching your children very early on about media, how they are done, what their goals are. Teaching them that commercials are lies and are made to sell...