All child object are even part of the rigidbody. I guess you have no other way than go the hierachy upwards and search your rigidbody. But you only have to do this once at start. You should save the reference in a variable.

JS or C#? it's always the same... (you should be forced to specify your prefered language when posting a question :D)

I guess you use JS?!?

So there are some different ways, but that depends on your setup. Normally rigidbodys should not be a child of something else because that would messup the physics. So to find the root GameObject of your sub-gameobject you can use Transform.root which returns the topmost gameObject in the chain.

var ownerRigidbody : Rigidbody;
function Start() {
    ownerRigidbody = transform.root.rigidbody;
}

function Update() {
    // use the velocity here: ownerRigidbody.velocity
}

If you have your rigidbody as child object (maybe just for grouping) you could search the rigidbody by going the parent chain upwards:

var ownerRigidbody : Rigidbody;
function Start() {
    var current = transform;
    while (current != null && current.rigidbody == null) {
        current = current.parent;
    }
    if (current != null)
        ownerRigidbody = current.rigidbody;
}