Fix sub group export to export direct children

What does this MR do?

There is a bug with sub groups export, where instead of exporting direct group children recursively, it would export all descendants of a group recursively, causing duplicate group entries in the JSON.

This MR fixes that issue, changing descendants to children & adding extra spec to cover this.

Renaming won't cause any issue as the feature is not yet released.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content