Do i need to make children of rigidbodys rigidbodys aswell?

On unity’s Physics Best Practices page (Physics Best Practices - Unity Learn) it states “As you can see from the figure, a total amount of 2000 warnings are generated, one for each moving game object. Also the average amount of CPU spent on Physics increased from ~17.6ms to ~35.85ms, which is quite a bit. When moving a game object, it’s imperative to add a RigidBody to it.”

If i have two boxes (boxA and boxB) and boxB is the child of boxA and boxA has a rigid body.

When i move boxA its going to move boxB, but will boxB need a rigidbody (in terms of performance) for the movement coming from its parent? Or would it only need a rigidbody if i wanted to change is local transform?

No, boxB will not need a rigid body to move with its parent. If the child object does have a rigid body then it will move freely from its parent, but if it does not then it will move with its parent.