PIM-2786 : create a dedicated getChildren method in the category tree controller

| Q                    | A
| -------------------- | ---
| Bug fix?             | N (extensibility point)
| New feature?         | N
| BC breaks?           | N
| CI currently passes? | Y
| Tests pass?          | Y
| Scenarios pass?      | Y
| Checkstyle issues?   | N
| Changelog updated?   | N
| Fixed tickets        | PIM-2786 (partially)
| Doc PR               |