Display specific rows and values in straight table

[daria]

Hi!

I have a parent-child relation and single rows in my table. 

I want to display only parent and single rows in my table ( marked as "+" in screenshot below).  So I set an expression for Child_Id column: If(Child_Id=0,Child_Id).

It works correctly but I have the next case: 

When parent has only one child then Instrument column of this child should be shown for this parent (instead of parent's value in the Instrument column).  Please see the last two rows on the screenshot below. I need to display the row with Parent_Id = 3720 and with Instrument = 'instr9'.

Do you have any ideas on how to do it?