Double Row Line Items
Could you please explain it in detail.? Any screenshot or excel template for what you trying to achieve would be helpful.
FYI, You can have child and parent in the column but, not as merged cells.
Not a lot of detail, but if you are trying to get this on a dashboard you could just create two different views, one at the Parent Level, one at the Child Level and setup synchronized selection.
If you are just trying to concatenate all Child values to the parent level, you could use TEXTLIST0