Rotate an object around one of his childs in the self space (not the world space)

I’m Rotating an objet around his Child like that :

transform.RotateAround(transform.GetChild(1).position, Vector3.back, oneSpeed * Time.deltaTime);

But the problem is that the RotateAround function does not have an overload to set the Space to Self (space of the child or the parent, dosn’t matter, it’s the same) instead of World.
I tried with RotateAroundLocal but I’m forced to set the point around which it rotates outside the function, like that :

transform.GetChild(1).RotateAroundLocal(Vector3.back, oneSpeed * Time.deltaTime, Space.Self);

And it only rotates the child, and not the parent.
Same tries with the Rotate function, it’s the same (RotateAroundLocal is deprecated)

Do you have an idea how I could do that ?
I’m a bit in trouble, i just realised 2 days before i have to present the work that it rotated in the world space…and have only until thursday to make this work.
Any help woul’d be appreciated.

Try this:

Transform t;
Vector3 v;

float oneSpeed = 50;

void Start(){

	t = transform.GetChild(1);
	v = transform.InverseTransformPoint(Vector3.back);

void Update () 

	transform.RotateAround(t.position,v, oneSpeed * Time.deltaTime);

Seem to work like that :

Vector3 pos = transform.GetChild(0).position;
transform.Rotate(Vector3.back, 5, Space.Self);
pos -= transform.GetChild(0).position;
transform.position += pos;