For any object if you want to know child relationships associated to it?
Check following snippet

Schema.DescribeSObjectResult R = Account.SObjectType.getDescribe();
for(Schema.ChildRelationship cr: R.getChildRelationships()){
    system.debug('Relationship Name =>'+cr.getRelationshipName());
}

NOTE: Please replace Account in first line with sObject which you want to get child relationship names