I think I've found a solution:
- Delete column associated with "Child Group" but retain the group
- Select "Parent Group" column, insert column within group - Right
- Set the value of this volume to "Child Group"
- Delete column associated with "Parent Group" but retain the group
- Select "Child Group" column, insert column within group - Left
- Set the value of this volume to "Parent Group"
This resulted in something similar to:
Then I:
- Selected the "Data 1" cell, selected add total after
- Merged all the cells in the row and added my data
- For the "Child Group" I selected the "Hide Duplicates" ("Child 1") option
- Then set the top and bottom border of the "Child Group" to none
which gave me: