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

| Q                    | A
| -------------------- | ---
| Bug fix?             |
| New feature?         |
| BC breaks?           |
| CI currently passes? |
| Tests pass?          |
| Scenarios pass?      |
| Checkstyle issues?   |
| Changelog updated?   |
| Fixed tickets        |
| Doc PR               |
integer or boolean ?