I’d say listen very carefully, to see whether she’s looking for reassurance or whether she’s ready for the next phase. Keep your tone neutral if you’re asking a question (what made you think that? Tell me more about that). Ideally I’d say wait until she asks you directly, but if there are younger siblings it could be wise to subtly elicit the conversation at a time when it’s just you and her and it won’t be overheard by little ears. Don’t be too direct; bring up Christmas or sort out some Christmas decorations, tidy the attic; that sort of thing.
My personal view is that it’s a developmental stage to be celebrated. Listen to her logic, and support her emerging wisdom. It’s very clever to work out the mystery. I don’t believe in lying; weaving the magic is one thing but when dc come to you to fact check I think it’s wise to be a solid source. Mine trust me to tell it straight and will check playground rumors out with me too.
I told each of mine that they were correct that santa wasn’t one single man, that actually he was more like an idea. We talked about the St Nicholas story and how people all over the world try and do nice things for others at Christmas, and how mums and dads all over the world work together to make Christmas a very special time.
And I told them that now they were part of the Christmas magic too. I told them it was a big secret but one they could share with (and I named) the adults in their life. But they had to be careful not to tell other children, and even if someone is wondering they should tell them to ask their mum, that it’s a conversation for mums!
I also made it clear that Christmas would go on as before, that now we’d play “the santa game” and they could help me. I’ve had this conversation with both my ds’s and they haven’t realized that the other one knows. Despite being awful to each other in other ways they have kept this secret faithfully.
I’m not very good at the whole fairies and magic business. And I won’t lie that it was a bit painful for all concerned. Ds1 came to me at bedtime for a cry, DS2 came questioning small details for months after and I wished I had the capacity to keep the magic alive for them.
But it has been good in other ways. It was so much less stressful than other christmases where I’d be hoping other adults wouldn’t give the game away, or that I’d slip up, or that there would be a last minute letter to santa, etc. I don’t have to censor movies why every Christmas movie has to question the existence of the big man is beyond me or be vigilant about books little house on the prairie nearly slipped through and they understand that Santa won’t be bringing them a LEGO Death Star anytime soon.
I enjoyed the Santa years and went all out with little details but I’m also enjoying this new stage. Last year I was able to involve them in the charity side of things. Before this I wouldn’t have wanted them to realize that some kids get nothing because it undermines the story. I like seeing their thoughtfulness and consideration. It’s still part of the magic of Christmas but in an older child’s version.
Hope this helps a bit, though I fully acknowledge that I’m a bit hard headed and their might be a better way for you. I’m not trying to tell you what to do, but maybe just offering a little reassurance that it’s not bad on the other side!