When did the dream of Santa die for you?

I realize that it's not realistic that my kids will believe in Santa forever. I know this. I really have been in some serious denial that my kids are inching closer and closer to that point where I have to explain to them what Santa is and how to still celebrate Christmas and not feel like you've been cheated. Does that make sense? I have no doubt that my kids know what Christmas is really about and how to celebrate it with a bright and giving heart. No doubt.

I'm still sad.

Here's the thing: a few weeks ago Olivia mentioned in the car, on the way home from school, that some kids in her class were talking about how Santa wasn't real. Thank god I was driving because I don't think I would have been able to keep a straight face. I very calmly asked her what she thought about it, and she told me she still believed.

But my heart broke a little bit. Because I know my time with her like this is coming to an end.

So this year I'm trying to just soak it all up with her. I think even with all of this, she's doubting everything. I do know this is the last year for all of this with her, I just know it. So now my next issue is how to explain to her about Santa and not have it be a let down for her. Does that make sense?

My fear is that she'll say or do something and ruin it for Jackson. I don't think she would do it on purpose, I really don't. I just feel a little brokenhearted that kids really do grow up. It feels like once a kid doesn't believe in Santa it's like a little piece of childhood dies.

So here's my question for you:

How do I explain Santa to Olivia without ruining Christmas for her, and how to get her to not feel like it's not as fun? 

I feel like this is just the tip of the iceberg, so to speak. Like this is the first tough conversation we'll have with her so I feel all of this pressure to get it right. This is the kind of thing they don't explain to you in those parenting magazines or What to Expect books. Diapers and teething are nothing compared to this kind of stuff.