qt - ChildrenRect always returns 0

Question

I have a Flickable item in where I want to put a customized flow component, so I created the Flickable like this:

import QtQuick 2.0
import UICore.Layouts 1.0 as Layouts
Flickable{
    anchors.fill: parent;
    contentWidth: parent.width;
    contentHeight: flow.childrenRect.height;

    Component.onCompleted: {
        console.log("The content height is: " + contentHeight);
    }


}

Then in my main.qml I have this (The file above is MyFlickable, and the MyFlow is a flow with specific properties):

 MyFlickable{
        ....
        MyFlow{
            id: flow
         ... 
        }
  }

This works great but the problem is that I want to be able to reuse this item with as little overhead as possible. Setting the id inside of my MyFlow doesn't work, and I tried this

contentHeight: contentItem.childrenRect.height

as suggested in here under contentWidth section: http://doc.qt.io/qt-5/qml-qtquick-flickable.html#contentItem-prop

but that always returns 0. I've tried a couple of other ways to receive onCompleted signals but I get the same luck. This doesn't work for example:

contentHeight: children[0].childrenRect.height

Which in my mind should be the same as accessing the item through the id, bu apparently not.

So my question is: how do I get to height of my flow after all the components have been added?

Thanks in advance!