I think the answer is to write Wikipedia articles in an in-universe style.
When I was a little kid, I really liked fantastical fiction, including cartoons of course. I think the cynical attitude that people take towards a lot of cartoon shows is that they exist only to sell toys or games. While this may be true to the people who run the companies, the existence of tie in merchandise gives the kids something to play with, a way to bring that fictional world into their own, which is a magical sort of thing.
Now say you're one of today's youth who wants to know more about something. What're you going to do? Go to Wikipedia of course, where you find an entry like this one about Krang. Maybe your reading comprehension is really high for a young kid, but you still might miss things that point out that this character is fictional. You may catch the things, like the heading Biography, that seem like evidence for this character actually existing. Even if you KNOW fiction is fiction, a Wikipedia article like this at least creates a sense of incongruity, which is interesting for a kid. Also, confusing kids is a classic path to hilarity.
Anyways, enjoy this footage of Krang talking!
"Because I enjoy seeing both people and animals suffer, and you my friends are both."