De man htop
:
INTERACTIVE COMMANDS
Space Tag or untag a process. Commands that can operate on multiple processes, like "kill", will then apply over the list of tagged processes, instead of the currently highlighted one.
U Untag all processes (remove all tags added with the Space key).
F9, k "Kill" process: sends a signal which is selected in a menu, to one or a group of processes. If processes were tagged, sends the signal to all tagged processes. If none is tagged, sends to the currently selected process.
Não é bem a resposta que você estava procurando, mas próxima. Você também pode eliminar grupos de processos ou filhos com kill, veja: