MY THOUGHTS EXACTLY
I think he was hired to kidnap them because the kids showed a lot of martial arts potential at that little tournament.
Don't quote me on that though, I'm not entirely sure.
He got his ass handed to him by Noh and cant take the shame of having been beaten by a first class warrior. It was never about the kids, he needs to kill the father so that he can go back to the Murim with his honor intact. As it stands now he can't return and tell them he failed his mission because he lost to what amounts to a common foot soldier.
Ahhhh, thank you!
Can someone explain why that old dude took the kids hostage??