My mother told me the "truth" when I was in 3rd class (so about 9) becuase she was afraid I was getting teased at school (there had been a question in about Septembre about where was Lapland on the map in geography, and I had answered "up near the north pole where Santa keeps his reindeers" to much hilarity). But I kept it a secret from my bro's and sis's until the youngest was much older (I think we first discussed it when he was 21!). That said, ANYONE sleeping under my parent's roof on Christmas Eve still has to leave out their socks for fruit, sweets and a book from the elder Mr Claus still in residence (and any parents in the house look after their own children's stockings themselves, the elder Mr Claus's grandkids).
In our own house, DD has a birthday during the Christmas season so it's major. And I am a Christmas nut too. So while we don't exactly lie, we do help her believe in the magic of Christmas. And I look on it as magic too - even if I am the one filling the stockings. She understands that different people believe in different things (multi-denom school and lots of different beliefs in friends). And everyone can celebrate with their friends (like recent Diwali celebrations) even if that is not their belief.
But specific traditions are:
Ater the youngest in the house lights the Christmas candle, DD (only child) puts out her stocking, carrot for Rudolph, milk and homemade cookies for SC, and has a bath and new PJs. (HM cookies are time dependent - I always have the oven on that evening anyway and some years we go from scratch to make the dough and fill time, other years I slice off a few from a frozen log of dough always in the freezer at that time of year for last minute treats!). And SC isn't allowed a beer cos then he wouldn;t be able to keep driving the sleigh!
While we read all sorts of Christmassy stories (Santa, Bible, general magic of the season....) throughout December at bedtime, on Christmas Eve itself we always have the first reading of "Twas the night before Christmas" of the year (I keep that particular book in with the Christmas decorations).
The stocking always has "something you want, something you need, something to eat and something to read". A large present, a few fun little things, something useful like socks or vests, a book (or 2) and both fruit and sweets (I try to get chocolate coins but don't always get around to that).
SC never wraps his presents, and they are always downstairs. Mom and Dad also have a present for DD under the tree, just as they have something for each other and she has something for them too (all opened later in the day).
We had an Advent Calendar last year, that my mum made, so I have to think about what goes into that this year. But we also do things together like presents for local charities to help SC make sure as many kids as possible get nice things for Christmas.