Yeah, Gang-du pulls his daugther and the boy out of the creature after it's killed. His daughter is dead, but the boy is alive. He realizes that the boy had been trapped with Hyun-seo down there and decides to take him in as his own.
I just thought it was great that they broke convention like that. Everything about this movie goes against the grain and does things you don't expect, breaking traditions, taboo, and cliches in the process. Once the creature is killed, everybody expects a traditional warm-fuzzy ending for the whole family where the kids are both safe and sound and everything goes back to normal. But it doesn't happen that way. One of the kids does survive, but the little girl we've been following since the start of the movie doesn't. I love it.
So the creature is defeated, Gang-du gets to continue being a father, and the boy gets a home and a family at last. It ends up that it is a happy ending for everybody, except for the little girl, who died.