All great guesses. The answer is: less than the total attack bonus for a device designed to be a minor threat to high Paragon Tier heroes.
What could only hit the players on a natural twenty could only miss the kid on a natural one.
Luckily, one of the other players caught that one before they followed through with Plan A.
Oh, and in other news, that Dragonborn that was arrested, tried, convicted, and sentenced? He managed to escape. Despite being hampered by chains, guards, odds, his own stupidity, a shark, and a Chuul (not related to Pinchy, of course) he managed to escape into the coastal wilds. He currently has 5 of his 12 healing surges, he's low on HP, he's wearing a muzzle that keeps him from opening his mouth wider than an inch or so, and he's still dragging his chains and manicales around. He's also still in the country he was convicted in, which is the country that the other PCs need on their side to farther their own goals.
As much as I hate the character, he and his player are somehow entralling enough that they convince me not to outright kill them. I feel compelled to make it work within the story rather than DM's will manifested, and so far he has overcome the story events I've thrown against him. I'm honestly very impressed.
But I'm still gonna try and kill him again next week.