Hi, I would refer to the following cross-post for the solution. 

• Solved: Re: Get task_run_id (or job_run_id) of a *launched... - Databricks Community - 153999

As @emma_s points out, it basically boils down to:

1. Pass {{tasks.parent1.run_id}} to a downstream notebook via base_parameters
2. In that notebook, call get-output with that ID → gives you run_job_output.run_id (the real parent1 run)
3. Call get-run on that → find child1 in the tasks list → grab its run_id

Basically, the part I was missing was getting the `run_job_output.run_id` with which to programmatically get the child1 run_id.