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//Querying all parent records
for(Parent__c parent: [SELECT Id,name,Status__c FROM Parent__c])
{
    if(parent.Status__c =='Completed')
    {
        //If parent status is Completed querying all child records.
      List<child__c> childList = [SELECT Id,Name,Parent__c, child_Status__c FROM Child__c WHERE Parent__c=:parent.id];
        for(child__c child:childList)
        {
            //Changing child status as completed and updating record.
            child.child_status__c ='Completed';
            update child;
        }
    }
}