J.R.
If I were in your shoes (and I've had differences of opinion with my daycare providers in the past), I would definitely let her know she'd crossed the line. I'd ask to schedule time with her away from the children so you can talk with her one-on-one with undivided attention. Let her know that she upset your daughter greatly with her note, and that she crossed the line as far as you're concerned. While you appreciate the love & care she has given your children, it is absolutely not her place to insert herself into the more intimate aspects of your lives - holiday celebration included.
Honestly, it goes beyond just sending the letter saying they've been bad. Not to put any ideas in your head, but if she decided that telling your daughter in writing that she hasn't been so good this year, albeit disguised as Santa, what does she say while your daughter is in her care?
I'd probably leave the "look what all we've done for you" out of the conversation unless you felt it was entirely necessary given the direction the discussion is going. I wouldn't let it go though - I'd definitely talk with her sooner rather than later.
Of course, I'd probably also find a different provider. We ended up finding another provider when I felt we'd butted heads one too many times with our first daycare director. The daycare was great with infants & toddlers, but not-so-hot with a bright preschooler.
Good luck. This is definitely a sticky situation.
J.
Mom to Chase (4) and "Sunny" (due 6.2.8)