Concurrently update the same bdm entry
I'll suppose that your child processes B and C are being called as subprocesses from the Parent process A.
In that canse, you can map data from the parent process, making it available in the called process. Specify data mapping when you define a call activity that calls a called process on the Execution tab - Data to send. You can pass the persistence Id of your BDM data and then run a query to select it in the subprocess.
Hope this helps