Mine have never 'believed', in the sense that they have never been encouraged to believe in the 'reality' of Father Christmas.
Father Christmas is a lovely story that we tell about Christmas. It's based on a very lovely older myth, which again is based on some evidence of a historical figure. In that way, it's like the Nativity story, and in the same way, some people believe it is literally true, some say it is a story with a deeper meaning, and some don't have it in their tradition at all.
We tell the story, and do the stockings, and talk about Father Christmas, and track him on NORAD, but we do all know it is a story - make believe is fantastic, as it is when you 'are' a train driver or a football star or a cook or a cat, and all the grown-ups in the family love to join in - and all that fun can be have without pretending it is real.
Some families like to make the story seem as if it is real for their youngest children, and it wouldn't be polite to tell them not to [tbh, this has never arisen - because there's never been a big mystery, or a big reveal, it's never seemed important enough to share]