Not all cartoons are for children but I would like to point out that cartoons have a wide appeal to children, regardless of if they understand the content or not. This is partially why people believe that cartoons are for children.
Case in point: my cousins have two children (right now they're six and nine). I'm not quite sure what they are in relation to me, but that's a moot point regardless. I visited them a couple of years ago in a family meeting in Hawaii, and at night their parents typically let them watch television while they're trying to go to sleep because it keeps them out of their hair and despite popular belief it actually puts them to sleep after a short while.
Anyway, as you can guess, Adult Swim comes on earlier in Hawaii, and although I really dislike anime I'd rather watch FMA than whatever crap happens to be on Nickelodeon. So while they were trying to go to sleep I'd be watching cartoons or anime (sometimes Robot Chicken would be on, too, which I guess is technically an animation, not a cartoon, but it's still unrealistic like a cartoon) with them... though I'd opt for Nickelodeon if and only if that stupid show about the kids that film movies came on. Obviously they did not understand what was going on, but because it was animated they liked it and kept quiet, eventually fell asleep, and it didn't seem to have any kind of impression on them.