[FEATURE] children action

__Tasks:__
- [x] implemented a unique key for categories
- [x] created a action which returns the children for a key
- [ ] gather feedback for my changes 

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | [none]
| Doc           | none
@marcelmoosbrugger please use $categoryManager => that would make the code cleaner!