Depends on the child, I guess
Not saying that the site is amazing for everyone, probably tweens and up should be allowed to roam freely is my thinking.
As a kid, I think that most of the Webtoons are pretty good! All the featured comics are free of explicit content, and when they do have triggers, there's always a warning in the beginning. I'd say kids fifth grade and up are probably mature enough to handle the content on Webtoon, but I suppose it depends on the kid.
Although there are themes of sex and drug abuse (ex. Edith and Boo It's Sex) there are never any (for lack of better word) 'weird' pictures. Plus, I don't think there are any messages that condone anything wrong, even Edith, which shows the smoking as problem for the main character and not as something a child should look up to.
All in all, I think that Webtoon is a great app with a very very wide range of genres and a lot of diversity. I'm not a parent, but I say you shouldn't be actively hiding teens about things like drug abuse, as it's a very real phenomenon and you're going to have to have 'that conversation' at some point, and having them know a little more might not be a such as bad thing, as they'll be informed for later life.