This is a first world story were hostages seem important be cause we care and we thick we got him now. This will not slow down the “righteous” cause of the Caliphate, especially since he is divorced from the mom and the child is a girl. If they are harmed he may eventually get around to kill the people did it but its not likely.
In America neither mother nor child could probably be held.
Detain a child because his father is an enemy leader/criminal?
We don’t do that, and I’m glad we don’t.