"children's cartoon shows"
Granted their intentions are dubious, there's no need to illegitimate a fandom because you don't appreciate it. Anime is marketed for ages up to 35 and covers a wide range of genres not appropriate for children for reasons ranging from violence and sexual content (hentai gore etc) to slow moving plots and novel based stories which children would find boring or would not understand. I doubt that Grave of the Firefly's could be classified as a children's cartoon show, neither could Monster, Mushishi, and the Ghost in the Shell is really marketed at older teens to those in their late 20's.
You may not have even heard of most of these and that is probably because US TV doesn't think they should bother showing anime that is more for an older crowd that can appreciate serious themes. Part of that is because they know that people like you will turn on the tv, see animation, and immediately classify it as a children's show and switch. I'm going to guess that you're either someone in their late 20's or above and that you've only been exposed to things like Naruto, Sailor moon, Pokemon, etc and had an entire childhood of cartoons for kids.
I am not going to ask you to research or explore this beyond your exposure, but I will ask that you please not make an uninformed generalisation about a whole medium based on maybe filtered exposure to one of it's genres. It would also be nice for you not to be a dick.
Now get off my lawn kudasai