public with sharing class myClass {
private final Parent__c SiteP;
public List<Child__c> childList {get; set;}
public List<GrandChild__c> grandchildList {get; set;}
public myClass (ApexPages.StandardController stdController)
{ SiteP = [SELECT Id, Name,Status__c FROM Parent__c
               WHERE Id =
:ApexPages.currentPage().getParameters().get('id')];
}

//Public lists here for Child and Grandchild records//

public PageReference saveAll() {
 update childlist;
 List<GrandChild__c> children = new List<GrandChildren__c>();
     for (Child__c c : childList) {
         grandchildlist.addAll(c.GrandChild__r);
     }
 update grandchildlist;
 update SiteP;
 return apexpages.currentpage();

<apex:page standardController="Parent__c" extensions="myClass">
 <apex:form >
    <apex:commandButton value="Save" action="{!saveAll}"/>

//Parent__c section here used from standard controller//

//Child__c code section here used from extension//

//GrandChild__c code section here used from extension//

 </apex:form>
</apex:page>