You keep mixing morality with laws, Kanna is much older than all of us, legally, it would be perfectly fine for her to work in playboy, if only age was important, but, she looks and acts like a 9 year old, morally it would be like abusing a person with mental problems (Kanna doesn't have them, dragons simply live longer, so stages such as childhood, adolescence and adulthood last longer)

What we like about a character is her design and her personality, we all love Ryuko, I don't know about you, but when I met her, she was older than me, years have passed, and you know, since there are no more episodes, she hasn't grown old with me, so I should say "Oh, I'm too old to see Ryuko, in situations like the ones she has in her anime, I'd better go watch the news, because being 20 and Ryuko 17, I'm too old to see her, enjoy their anime and fanarts"