The following has been stolen from here.
Insert the following into your custom module.
/**
* Implements hook_node_delete().
*/
function mymodule_node_delete($node) {
if ($node->type == 'mynodetype'){
$nids = db_query('SELECT entity_id FROM {field_data_field_review_reference} WHERE field_review_reference_nid = :nid', array(':nid' => $node->nid))->fetchCol();
node_delete_multiple($nids);
}
}