One word: Training.
An old man trapped in a room won't likely be the last Westerosian that Qyburn wants dead. It's now clear that his approach to scaling his organization is by using destitute children. Which makes sense, (once you get past any moral revulsion you might have): they're malleable, desperate for someone who cares about them, and aren't considered a threat by others.
Qyburn's developing a team, and we know that the teams' goals include:
- Spying on Qyburn's - or Cercei's - frenemies
- Killing their enemies, including some that likely can't be dispatched via mass-wildfyre-bombings
So, if you want to turn kids into assassins that can take out real threats someday, you'd likely need to do two things:
- Let them practice the physical skills involved, and
- Desensitize them to the natural moral resistance, or their fear of new things
Both of those are achieved by making them start with victims that pose no threat in controlled, supervised environments.
(If he's read the hot management tomes of the era, I assume he's already had them watch a mass stabbing prior to this scene's events, and will encourage them to teach a mass stabbing to the next class of urchins to really lock it in.)