parent:                    child1:                             child2:
+-----------+----------+   +----------+------------+-------+   +----------+------------+--------+
| parent_id | name     |   | child_id | date       | color |   | child_id | date       | lenght |
+-----------+----------+   +----------+------------+-------+   +----------+------------+--------+
|         1 | Bob      |   |        1 | 2017-07-20 | red   |   |        2 | 2017-07-21 | 185    |
|         2 | Peter    |   |        3 | 2017-07-24 | blue  |   +----------+------------+--------+
|         3 | Alice    |   +----------+------------+-------+
+-----------+----------+

combined desired result:
+-----------+-------+----------+------------+-------+--------+
| parent_id | name  | child_id | date       | color | lenght |
|         1 | Bob   |        1 | 2017-07-20 | red   | NULL   |
|         2 | Peter |        2 | 2017-07-21 | NULL  | 185    |
|         3 | Alice |        3 | 2017-07-24 | blue  | NULL   |
+-----------+-------+----------+------------+-------+--------+