Ever since the kids were little (I was a single parent) I would buy myself something little for in the stocking and then I'd thank my kids for it. Every year I laugh at the looks on their faces.
Trick is to use a different colored wrap paper so they don't associate this gift with matching the ones they know you bought.

Then they'd say, honest mom, we didn't get you that, I'd pass it off as if it wasn't you it has to be St Nick or the Christmas Angel.