My mother told me (the eldest). (In fairness, it was to do with, on being asked a question in a combined y3/y4 class about "where is Greenland?", I replied up near North Pole where Santa keeps his reindeer, and she didn't want me getting laughed at even more). Half the class still believed in December anyway, just were waaay more cynical in September (and tended to be cruel to me anyway as the "outsider" from the village as our family was not able to trace back 6 generations - we moved there when I was less than a year old). I was then on reduced rations (half a fruitbowl of fresh fruit to yourself!! (no row over who got the 1 red apple, as everyone got 1), sweets, book, I think there were a few small bits that year but no longer a big present) - but still had to leave out socks as the younger ones still believed.
She didn't tell any of the other 5 DCs when it came time, she let them work it out themselves. Suffice to say that as youngest DB (now aged 30) has never publicly renounced his belief, it allowed DPs to continue to fill any pairs of socks left out on Christmas Eve with a half a fruit bowl each, sweets and an age appropriate book. There were 3 filled last Christmas!!
I am convinced that DD (almost 9 and extremely logical) has worked out both FC/TF at least by last Christmas. But she's not saying anything. She's an only child, and when it becomes clear that she is a non-believer, we will continue to have stockings but DH and I will get one too then and she can help fill those 2.
I see it as part of the magic of the season and the spirit of giving. DD already goes and chooses a present to buy with money I give her that she would like but that we give to a local charity which helps children otherwise getting nothing, for someone her age. She wants to put things on the tree early in the morning while DH is not looking, so he gets a surprise from Santa too (last year, she had bought a Yorkie bar, as they are "not for girls" so would obviously be for DH!).
Just as in my house growing up, DD always has something under the tree from us. And any presents from anyone else in the wider family have always been from those people.
Santa brings a main present (or 2 smaller mains), and a stocking with fun stuff, half a fruitbowl, sweets, a book, and a few things needed too (socks, undies, toothbrush, facecloth, hair bobbins etc types of things I'd have to buy anyway). So he is both fun and practical. And we tend to focus more on the Christian story (even if I am practically atheist) and nature, and seasonal magic generally, rather than Santa, elves and being watched over for being good (although that has crept in on occasion) - and we don't do Elf on a Shelf or anything like that.