[kagaw]

Kristoff is brainwashed. Remember when he saw the trolls and the royal family? A troll show up beside Kristoff and said "Cuties! I'm gonna keep you!" (That's the weirdest line, I've ever heard in that movie). After that, the troll do its magic brainwash to Kristoff. The trolls motive is to keep their power in secret.

And Kristoff is just a kid, it is possible that his father is one of the iceman(the opening scene cutting ice).

[huffmsa]

I did think it was weird that they abruptly adopted him. He didn't seem like an orphan.

[atomicnumber3]

In Disney movies, that's easily remedied.