Debug multi-process application problem

I'm trying debug multi-process app. So, I do it:

1. Start Debugging (F5)
2. Type in Debug Console -exec attach <pid>, <pid> -- is process id of child from ps aux | grep appname
3. However, I get error:

-exec attach 3331
Unable to perform this action because the process is running.

4. So, I guessed that I should set debugging to pause. Pressed Pause (F6), and got following error:

-exec attach 3331
A program is being debugged already.  Kill it? (y or n) [answered Y; input not from terminal]
...
Thread 1 "kamailio" stopped.
0x00007ffff76a9912 in __libc_pause () at ../sysdeps/unix/sysv/linux/pause.c:30
ERROR: Error while trying to enter break state. Debugging will now stop. GDB exited unexpectedly.
The program '/home/marat/work_folder/kamailio-git/build/sbin/kamailio' has exited with code 42 (0x0000002a).

ERROR: GDB exited unexpectedly. Debugging will now abort.

-exec set follow-fork-mode child is not suitable I think, because application has more than 1 child processes.
And how I can fix it?

Start by reproducing the F5/F6 workflow in the Debug Console with -exec attach <pid> against an application with multiple child processes. Investigate how the VS Code C/C++ extension and GDB handle attaching while the launched program is running and entering break state. Done means multi-process debugging no longer exits GDB unexpectedly, or the supported workflow and limitation are documented.