What can you do if the mother will not allow the children see or talk to their father abruptly?

It depends on the custody order already in place. If the mother is violating the custody order, the father can sue her for contempt, and ask that she be appropriately sanctioned.

If there is no custody order in place, the father will need to sue the mother for legitimation or divorce, depending on whether they are married, and ask that custody be determined.