Disowning child from parent process

If $PID_PARENT launched $PID_CHILD, how can I detach (disown?) $PID_CHILD from $PID_PARENT so that, when I kill $PID_PARENT, $PID_CHILD keeps running like nothing happened? Concretely, my $PID_PARENT is the process running Jenkins (or the Java process that runs the server that runs Jenkins) and $PID_CHILD is a _really long_ job that I don't want to have to restart after restarting Jenkins (which is needed for some maintenance). Essentially, I want to stop Jenkins but not stop the long job it started and I know both PIDs. **UPDATE 1:** I found out about disown and tried from a login shell (so not the parent PID shell):

-shell
disown $PID_CHILD

but got

-bash: disown: 13924: no such job

The $PID_CHILD correct and doing

-shell
ps -o ppid= $PID_CHILD

returns $PID_PARENT **UPDATE 2:** As per @Rui's answer , I made a temporary hack job in Jenkins that only runs from the parent shell this time:

-shell
disown 13924

but still got

disown: 13924: no such job