child/main*> select * from (select 'parent' as db, * from parent.t union select 'child' as db, * from child.t) as combined;
+--------+----+-------+
| db     | id | words |
+--------+----+-------+
| parent | 1  | foo   |
| parent | 2  | bar   |
| parent | 3  | baz   |
| parent | 4  | zap   |
| child  | 2  | bar   |
| child  | 3  | baz   |
| child  | 1  | foo   |
| child  | 4  | zap   |
+--------+----+-------+
8 rows in set (0.00 sec) 

mysql> select * from (select 'parent' as db, * from parent.t union select 'child' as db, * from child.t) as combined;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '* from parent.t union select 'child' as db, * from child.t) as combined' at line 1