If the node isn’t rotated, than its pretty simple to do:
relative pos=child.absoluteBounds.pos-parent.absoluteBounds.pos
size=child.absoluteBounds.size
but if the node is rotated, than the absoluteBounds reflects the overall space it takes instead of the size of the node.
My trigonometry isn’t the best, but I don’t think that’s enough information to calculate the width and height of the node.
Is there anything we can do to derive this?