A woman I used to know told my dd at only five that there was no FC. I got my own back by telling dd that X had probably been so naughty when she was little that FC never came, so no wonder she didn’t believe in him. Should add that X had form for yelling/effing and blinding in public, as dd had witnessed, which certainly helped - it worked like a charm.
Having loved all the FC excitement as a child, I would have hated for it to be denied to my own children. And I must say it never occurred to me when older that my folks had ‘lied’ to me over FC or the tooth fairy. I was well aware that it was rather a case of not denying me that little bit of young-childhood magic and excitement.