How do I automatically create a comment on a Feature based on the status of its children

[Phil Hind]

If  a Feature is transitioned to "Done" but it has children that are still "In Progress", I would like to transition the Feature back to a previous state and add a comment that it can't be "Done" until all its children are "Done".

I am struggling to put an automation together as I seem to need to go

Trigger: Feature (transitions to Done) - > Condition: Check Children (status) -> Action: Feature (transition back)