How to kill all subprocesses of shell?

Question

pkill -P $$

will fit (just kills it's own descendants)

EDIT: I got a downvote, don't know why. Anyway here is the help of -P

   -P, --parent ppid,...
          Only match processes whose parent process ID is listed.

and $$ is the process id of the script itself

After starting each child process, you can get its id with

ID=$!

Then you can use the stored PIDs to find and kill all grandchild etc. processes as described here or here.

If you use a negative PID with kill it will kill a process group. Example:

kill -- -1234

Extending pihentagy's answer to recursively kill all descendants (not just children):

kill_descendant_processes() {
    local pid="$1"
    local and_self="${2:-false}"
    if children="$(pgrep -P "$pid")"; then
        for child in $children; do
            kill_descendant_processes "$child" true
        done
    fi
    if [[ "$and_self" == true ]]; then
        kill -9 "$pid"
    fi
}

Now

kill_descendant_processes $$

will kill descedants of the current script/shell.

(Tested on Mac OS 10.9.5. Only depends on pgrep and kill)