why does viki only has 6 episode is it uncomplete?
It's just a different cut, I think. The 22 episodes listed here are only 10 minutes long, and the episodes on Viki are half an hour. (And as someone said below, if you cut out the music and credits on that many short episodes, that accounts for what appears to be missing time on the six-episode version.)
Could someone explain the children in the end to me is that supposed to be them reincarnated?
It's the two of them when they were little, before all the other memories we've seen. When he gives her lunch at school in their first memory he says something like, "Have we met before? I lived in Seoul when I was little." So in the ending, we're seeing that very first meeting.
10: https://www.youtube.com/watch?v=oU-OAgwr9v0