Go to the mother and add the father as if he were a spouse. Then go to the Person View for the mother and click the Relationship Tab. Select the spousal relationship for the father. The right hand pane will now show the father. Select the drop down menu for the relationship and you will be given a number of choices. Select the one that is appropriate. You can also enter the current status of the relationship. The father will still show as a spouse on many screens in FTM but the relationship in reports will be Met, not Married.
You can then enter or attach the child to this relationship. If the child is currently listed as a child of the first husband you will have to detach her from that relationship before attaching her to the correct one.