How can I access drag and drops x and y from a child element?

The doc says:

You can access the coordinates of the element when the user drags it as ‘X position’ and ‘Y position’.

But there is nothing to say how. Like dnd_x/dnd_y or whatever.

And since I need to read/write it from a child element, how do I do that when the child is already picking up thing data via the drag and drop group?