You give little by way of the facts. However, the general rule is that without some kind of court order in effect, one parent has just as much right to the children as the other. There can be no charge of parental kidnapping in such a case. Therefore, the father would be within his legal rights to go and pick up the children. However, he could not use force or even trespass do to so. At that point the police would have to be called or he would have to go to court and have temporary orders issued. Perhaps, now is the time to consult with a family law attorney as to all of this.