[SalesForce] How to get child relationship name from Parent Object in Apex

In the following piece of code, i am trying to do a dynamic parent – child query

 public static void fetchRelatedRecords(SObject primaryRecord){

    String recordId = (String)primaryRecord.get('Id');
    Map<String, Schema.SobjectType> keys = new Map<String, Schema.SobjectType>();
    Map<String, Schema.SobjectType> describe = Schema.getGlobalDescribe();
    for(String s : describe.keyset()){
        keys.put(describe.get(s).getDescribe().getKeyPrefix(),describe.get(s));
    }

    Schema.SObjectType sobjectTypeObj = keys.get(recordId.substring(0,3));        
    Schema.DescribeSObjectResult obj = sobjectTypeObj.getDescribe();        
    String query = 'select Id, ';
    for (Schema.ChildRelationship cr: obj.getChildRelationships()){
        system.debug('Child Object Relationship Name:'+cr.getChildSObject());
        query = query + '(select Id from ' + String.valueOf(cr.getChildSObject()) + '), ';            
    }
    query = query.removeEnd(', ');
    query = query + ' from ' + String.valueOf(sobjectTypeObj) + ' where Id = ' + '\'' + recordId + '\'';
    List<SObject> relatedRecordList = Database.query(query);
}  

My problem here is that, the line

system.debug('Child Object Relationship Name:'+cr.getChildSObject());

gives the child object Name, but what I want is the Relationship Name.

e.g. If Account is the parent of Custom object Author, i want the Name Author__r and not Author__c.

Is there any way to achieve that?

I tried using cr.getRelationshipName() as well, but that did not work either.