Thanks, I would greatly appreciate this functionality!
I'll briefly outline how we use the Group Stories / Child Stories in our org....
We have our typical User Stories written up with the user in mind: "As an XXX user, I would like to be able to YYY so that ZZZ"
Now in a vanilla setup, this User Story should suffice for the Team to do a Task Breakdown.
It's not so simple for us (Enterprise IT), due to the fact that our User Stories are not isolated to a single piece of software.
Our User Stories are impacting multiple different applications from Portal UI's down through different levels of Services (all owned by different teams) to maybe some SAP system, and back up again.
A User Story cannot be assigned to a single team for task breakdown, as there are multiple teams and applications behind that one User Story.
This is where the Story Group / Child Stories come into play for us.
We use the Break Story functionality to create Child Stories for each application affected by the User Story.
This way we get to have a "Real" backlog view (as any Group Story is per definition for us a User Story which is just the "what", not concerned with the "how"), ready to be prioritised by what brings the most value to end users.
I could provide some more information if anyone requires.
Hope this explains clearly how my organisation uses the StoryGroup/ChildStories elements!