I want to write an trigger,The condition is" if the parent case records should not be close,when its having a child case are open".It will show an error when i close the parent case,

trigger ParentChildcase on Case (before update) {
List<Id> parentCaseIds = new List<Id>();
Map<Id,Case> caseMap =new Map<Id,Case> ([select id  from Case where Id in:parentCaseIds]);
/*List<case> OpenChildCases = [select Id from case Id:Trigger.NewMap.Keyset() and status != 'Closed']
Set<String> ChildCaseSet = New Set<string>();
    for(case temp:OpenChildCases){
        ChildCaseSet.add(temp.Parent.Id);
    }
    for(case caseinst:Trigger.New){
      if(ChildCaseSet.contains(caseinst.Id) && caseinst.Status = 'Closed'){
        caseinst.addError('Unclosed Child Records are Present');
     }   
   }*/
}