Under each children's profiles, select edit, and you will see on the right hand side, "Edit Family". Under fathers name you will see (edit father). access this by clicking on it.
You now have the opportunity to add/replace or remove father. Hit the link "remove" and follow intructions.
Remember when you have completed this exercise, the child profile is free, and requires reconnection. Follow the same process but this time hit "add/replace" and follow instructions.