How to fetch the value from child table to another child table in Same Doctype Using Custom Button Click

hi all
could any one tell us How to fetch the values from child table to another child table in Same Doctype using Button Click that is added to the Doctype
the button name is trk_calc
so i want every thing in the Items child table to be copied to items_track child table within purchase order Doctype when this trk_calc button clicked.
thank you All