Using AnimationClip.SetCurve on child object.

If i have 2 cubes, the first is the parent of the second in the hierarchy. How do I aim the relativePath parameter at the child cube?

Here is what I have tried, the code is in a script attached to the parent.

 private void Test()
 {
     AnimationClip clip = new AnimationClip();
     AnimationCurve curve = new AnimationCurve.Linear(0, 1, 2, 3);
  
     clip.SetCurve("", typeof(Transform), "localPosition.x", curve);
     clip.SetCurve("Child", typeof(Transform), "localEulerAngles.x", curve);
  
     animation.AddClip(clip, "test");
  
     animation.Play("test");
 }

Obviously, this doesn't work. The reference(link), does not really shed any light on how to use the relativePath parameter.