Sometimes on Halloween when families are out trick or treating themselves, they'll leave a bowl of candy out in front of their house with a sign that says something like "take one per child".
This is a play on that, but it's a bump of cocaine per child. You're supposed to snort it using the key.
I think you just scoop the key tip in the bad to get a little ... bump... of that stuff. I have no idea on size of scoop. that likely correlates to purity or cut of goods.
Instead of a bowl of candy that kids would take one of on the night of Halloween, a US holiday, they are jokingly offering a small amount of a drug, likely cocaine, to the kids.