How do I make children independant from parent movement?

In my game I have undesired movement of children when moving the parent object. This happens when players are standing on a platform and it turns, the player turns with it (which is not what I want). The player also seems to build up force and when not parented anymore the force shoots them off.

I think that you have misunderstood how a scene’s hierarchy of gameobjects work.
If a gameobject is a child of another gameobject (parent), its transform matrix will be relative to that of the parent.
This allows you to for instance move all of the objects inside a house when moving the house itself.
In your particular case it seems like your player should really not be a child of the platform, and that instead they should both be children of yet another gameobject, let’s call this Level. This way, the movement of the platform will be independent from the movement of the player and vice-versa.

If you really need to do what you are describing for whatever reason, you probably want to save the Global position and rotation of your player before you move the platform, and then make the player translate and rotate its position and rotation to the global ones.

Have a look at the Transform documentation, in particular to the Local To World matrix operations.