The problem is that I'm already complicit in the deception. Besides putting quarters under her pillow, last night (Dec 24) I helped her mother put presents from Santa under the tree. Even before then, I was with her mother while she bought presents that were going to be (and eventually were) from Santa, and I knew all about it.

If somebody were fooling her about Jesus in this way, I would be a lot more worried. (I'm not sure how I'd intervene, which would depend a lot on circumstances, but I'd certainly want to.) But she'll find out about Santa soon enough; I justify it to myself as less important. Lying to a kid about Santa, like making honest mistakes when talking about Jesus, is raising a child differently from how I would (whereas as lying to a kid about Jesus, with the intent that they believe the lie forever, is a step beyond).

However, I do have an answer for questions from random children about the existence of Santa (which I haven't really tried out yet). And that is to quiz them about where they think that their presents come from, giving them a chance to figure out this answer for themselves. That's probably what I'll do here; the complication is that I know exactly where her presents come from (and by direction observation rather than by deduction from reasonable assumptions, as I would for a random child).

In any case, I don't think that she's likely to ask me for another year now.