Aside from some obvious points where it is actually little kid lovers who like a character because they're a kid, most people (at least here) are attracted to cartoon characters because of the character. Whether someone like Mandy is 8 or 80, as long as she has her monotone voice, blonde devil hair, and pink dress, they'll want to fuck her.
Most kid characters have aggressive adult personalities and are voiced by adults that you end up naturally thinking more of the character isolated from their personality compared to their age.
Take a look at Patti Mayonnaise. She looks like she's around 16 because of her design but she's actually around 12. yet, I bet most people were fine with fucking her two seconds ago when they thought she was 16, but nothing really changed because it's still the same fictional cartoon drawing of Patti. People just want to fuck Patti, not a 12-year old.
This is not universal, there are people who want to fuck the Loud sisters because they're underaged, but that's not always the case.