I would probably just shoot him with a gun honestly. Clearly fire doesn't work and I'm sure he'd survive a sword/knife. Or I'll expose him to a Popeye's biscuit, those things can kill a healthy young man.
If I had to pick a favourite kid... It'd have to be Jeremy. He's fun and bunnies are one of my favourite animals :D
But it's a close the with Cassidy aka vengeful spirit. They're just a cool little dude :)