I'm using the following code. It's in an on click event on the child row.
The code does change the two fields on the parent row, but they do not get saved. and change back to what they were to start with.

{grid.Object}.submitGridPartRow({Grid.RowNumber});
var po={grid.Object}.getParentObject();
var sl={grid.Object}.getValue('G','Store_Location',{Grid.RowNumber});
var st={grid.Object}.getValue('G','Stock_Number',{Grid.RowNumber});
po.setValue('D','Store_Location',sl);
po.setValue('D','Stock_Number',st);
po.submitGridPart();