LOL, I had told my kids St Nic was a dressed up guy. And that it was all about the game of giving eachother presents and pranks, while being unknown. It was not about a real St Nic. He clearly is not real. Told them that at a very young age. It never interfered with them believing in the game of St Nic. That was a firm belief till they were 10 or 8.
I had told them similar things about other traditions. But, had never said that specifically about the toothfairy. Thought it was obvious. Since they had expressed similar thoughts about God too.
So, one day I had kids over for lunch, and they started telling the toothfairy wasn't real. My kids were shocked, so I told them it was similar to St Nic, and that I had thought they had realised that.
At that moment the kids I had over for lunch started to cry, because I had made them realise st Nic wasn't real.