This features is included in #ConfigMgr 1710 release.

General Blog: https://cloudblogs.microsoft.com/enterprisemobility/?p=67486
Docs: https://docs.microsoft.com/en-us/sccm/core/plan-design/changes/whats-new-in-version-1710

The notes for the 1711 Tech Preview apply here too and we’ll continue to work on these

Console actions are fixed in many instances where we didn’t traverse the referenced packages/content
Export now exports child task sequence and referenced content – secrets are cleared from the referenced task sequences too
Task Sequence import now works and detects duplicates
Copy action copies parent and all reference to child task sequences
Delete action detects if there’s a parent child relationship and displays what’s affected
Similarly deleting packages that are in child task sequences is detected and shown in the delete package wizard
Initially we had limited to one level – now there’s no limit, but use that wisely
Related to the above, we do not permit a circular reference i.e. a parent cannot reference same parent, a parent cannot reference a child that references same parent – we traverse the entire chain
Create Prestage Content is updated to work for task sequences with multiple child references
Deployment Verification now detects when the child contains the ‘high-impact’ steps
Standalone media creation detects and adds all task sequence and content references and completes end-to-end

Things to note
We do not permit selecting a task sequence with a Boot Image reference, for any deployment that requires a boot image that reference has to be on the parent
Any chain containing a disabled task sequence will fail – the continue on error won’t work for that step
Criteria for popping the ‘high-impact’ warning are not detected in Software Center when the child contains the ‘high-impact’ steps, in this case our recommendation is to use the Task Sequence User Notification properties to force the ‘high-impact’ warning
If a child task sequence has a package reference removed the parent does not detect this. To workaround, make any edit to the parent (anything that activates Apply)

As always we appreciate any and all feedback, try it out and let us know what you think