Unfortunately, you have to move each child separately. FTM used to recognize siblings of a move and would ask if you wanted all siblings to be moved, but that function was removed after FTM16.
That said, here is what you do with each child:
1 Select a child - in either Family or Person tab.
2 Main menu > Person > Attach/Detach > Detach from Father and second wife.
3) Your selection should still hold on that child. Now go to Main Menu > Person > Attach/Detach > Attach to Father (or correct mother, it doesn't make any difference). Then, choose the correct father-mother combination that is the correct couple.
4) Click ok.
Now repeat for each child.
All subsequent generations will follow the move.