Unable to answer "Who is allowed to use this playground?" – Playground – https://osm.org/way/849278752 via StreetComplete 58.2:

This is a school playground, so while non-students should stay out during school hours, there are no signs nor fences asking anyone to keep out.
The playground is visibly designed for small children.

Attached photo(s):
https://westnordost.de/p/224280.jpg
https://westnordost.de/p/224281.jpg