public class accconhierarchy {
Private list<Action_Item__c> acc;

public List<accountWrap> accountList { get; set; }
public accconhierarchy (){
    List<contactWrap> cw;
    accountList = new List<accountWrap>();
    acc = [select id,name,Priority__c,Hours_act__c,Due_Date__c,(select id,name,Priority__c,Hours_act__c,Due_Date__c from Related_Action_Items__r) from Action_Item__c limit 500];
    for(Action_Item__c a:acc){
        cw = new list<contactWrap>();
        For(Action_Item__c co : a.Related_Action_Items__r){
           cw.add(new contactWrap(co));
        }
        accountList.add(new accountWrap(a,false,cw));
    }
}

public class accountWrap{
    public Action_Item__c oAccount{get;set;}
    public boolean isSelected{get;set;}
    public List<contactWrap> contactset{get;set;}
    public accountWrap(Action_Item__c a,boolean b, List<contactWrap> c){
        oAccount=a;
        isSelected=b;
        contactset =c;
    }
}

public class contactWrap{
    public Action_Item__c oContact{get;set;}
    public boolean isSelected{get;set;}
    public contactWrap(Action_Item__c a){
        oContact=a;
        isSelected=false;
    }
}
}