According to a documentation of addChild function, the return value must be boolean which denotes whether the item is added successfully.
However, the addItemChild function of CDbAuthManager (/yii/framework/web/auth/CDbAuthManager.php:123) returns nothing, thus eventually at successful insert the return value is null.
I highly believe it’s a bug. Temporarily I patched it like the following at line 155:
return $this->db->createCommand() ->insert($this->itemChildTable, array( 'parent'=>$itemName, 'child'=>$childName, )) > 0;
Let me know if this is correct.