Reminds me of "Kill Bill"...

Also... If I imagine myself in the child's shoes, it would make a difference whether my dad was murdered from behind (like they tried to do with Ethan) or was the attacker.

There may be a grey zone but right now I can't think of one.

Yeah... if you can disarm the kid (which isn't necessarily easy just because they're young), call the authorities and move on, that's one thing. But if this kid is old enough and determined enough to actually track you down for revenge? What's to say they won't do it again? Like you said, no pleasant answers.