Our 8 year old GD is facing this at school. She desperately wants to believe, but is writing a letter to Santa that she won't let her parents see 'as a test'. Apparently she did leave this on the table when distracted, and her mother spotted that apart from a Fitbit, she wanted an end to all wars, no more world poverty, and various other altruistic impossibles! She says she is going to email this to Granny (me) so I can send it off without her parents knowing what is in it. We have obviously explained that Father Christmas could not possibly give everyone everything they asked for.
I suppose I could say I forgot to post the letter, when she checks the news on Christmas Day!
I didn't have this problem with her mum and my other children, I told them the truth as soon as the very first questions started, in their case around 4! I said to them that there was a Father Christmas who lived a long time ago, he was called St Nicholas, and in the days when children used to leaved their muddy shoes outside the door, he would go round at night and fill them with sweets. He died a long time ago, but the desire to make children happy at Christmas time lives on in all of us, and we carry on the tradition in our own houses with stockings.
I was concerned that if left too late, there would be a 'baby and bathwater' situation, where if Father Christmas wasn't real, then they would not believe in the real Christmas story either, which for us is the real cause of joy on Dec 25th!