I can’t really think of harms I suffered as a child from watching ads. It probably made me want to buy more stuff but since I was a kid I had to ask my parents and they usually said no. I was slightly sad but then I got over it. So if anything it probably taught me to be a more discerning consumer that wouldn’t buy anything just because of an ad.
If we hide the real world from children too much then how do we expect them to learn to deal with it all at once? They will turn 18, get a credit card, and be bombarded with ads they haven’t learned to control.
Unless you want to argue that ads shouldn’t be shown to adults either (which has major free speech issues) I think it’s better for kids to learn to deal with them responsibly.