I know you've been torturing yourselves with wondering, since my last "secrets of the tooth fairies" post. How in the world are tooth fairies and the Black Death linked?
It's a long story. First, humans and the fae had been at war for a very long time. Humans were winning. They're better at killing, and they breed like rats*. The fae could see three options. Disappear into The Gloaming, make humans more like fae, or get better at killing humans. There was much debate (and it still hasn't been resolved) but finally a new fae was introduced. Oubliette, who had the power to kill humans with just a scratch. Oubliette (the tales aren't clear if Oubliette was one fae, a family of fae, or a new race) was incredibly good at her job. Humans were dropping like flies. They called it "The Black Death" and in later years humans wrongly assumed that it was a form of the bubonic plague.
The fae might have won the war, except for one thing. Oubliette did not discriminate between children and adults. Because fae take so long to have children, they are considered precious, and Oubliette's actions were horrific to the other fae. She argued that she didn't know they were children. That humans couldn't even tell the difference between children and adults, so how could she? The markers of adulthood (marriage, living on your own, etc) could happen to a human at any age. More debate and it was determined that once a child lost its "baby teeth" and adult teeth were growing in, it was no longer a child.
Oubliette took this to mean when a child lost its first baby tooth, and her kill rate didn't noticeably drop. Fae and humans finally signed a pact. If a human gave an offering of blood and bone to the fae, and received fae silver in exchange, they were promising not to harm the fae. They would be safe from Oubliette. Humans forget their pacts almost as soon as the ink dries, but they remember the important part, and give a child's first tooth to the fae. In exchange, fairy silver is left behind (which changes into what ever money the family expects to see.)
So, children are safe from Oubliette until they lose their first tooth. They are safe from Oubliette once they've given their first tooth to the tooth fairy and gotten money (or something) in exchange. But for that one night, the children are terribly vulnerable.
*This is why "rats" is a fae term for humans. Any fairy tale you've read, like The Pied Piper of Hamlin, that has rats in it, is really a fae story about humans killing fae.