You can detach a child, one at a time, from the Husband-Wife or detach the Parents from a child, again one-at-a-time.
You do this through the Menu by selecting either the child or a parent in the People Workspace:
Menu > Person > Detach > (parent or child).
Then, you re-attach the child to proper set of parents or parents to the right child at the same place:
Menu > Person > Attach (father, mother, child)
Note that FTM keeps track of parents as couples, not individuals. You would detach a child from Daniel-Alice and attach that child to Daniel-Mary.