You don't need to add a column to the tables. Just add it to the UNION.
If that's what you meant, then that is the correct way to do it.
Code:
SELECT 0 as tablenumber, `order`, FROM all_child_fields WHERE editible='1'
UNION
SELECT 1, `order`, field_name FROM a_child_fields WHERE editible='1'
ORDER BY tablenumber, `order`