DD (7) told a classmate that the tooth fairy didn't exist.
For me - not a massive deal, the tooth fairy is a secondary magical creature (not like Father Christmas) - only just more viable than a giant bunny delivering chocolate eggs, and surely no one believes in the Easter Bunny, not really.
For the parent of the other child (same age) it is a HUGE FUCKING DEAL - the sort of deal that has been mentioned around 20 times since. Usually in a sneering manner. The sort of big deal that means that she sniggered at DD when she was (painfully) losing a milk tooth (despite the fact that DD was crying because it was spiky and bleeding) - which is just not kind.
The bottom line is, DD is not easy to trick - she asked me earnestly, directly, and specifically said "please don't lie" ...so I didn't lie.
What the other mother doesn't know is that DD also knows about Father Christmas - but has taken the task of keeping that secret VERY seriously, she's not a blabber-mouth, we just didn't know the tooth fairy was a big enough deal for someone to give a shit about for months and months.