They shouldn't punish the kid. He wasn't doing anything wrong - he was being resourceful. I'm positive a kid in 6th grade doesn't know it's illegal to copy money. I tried it a bunch of times when I was around that age. Figured somebody had to "make" money, why not me?
I actually got caught when I was trying to buy some baseball cards. I went to that shop every Wednesday though and bought a pack, so the guy knew me. He gave me the pack for free that week, but said I shouldn't do that anymore cause I'd get in big trouble.
If the kids don't know, then teach them, don't punish them. And shit, he counterfeited TWENTY DOLLARS. He's no threat to anyone.