M.R.
If this is an in-home provider, I think it would be perfectly acceptable and not tacky at all to give a 'holiday bonus'. Many people who work in the private sector receive a holiday or end-of-year bonus, so as her employer you could treat the situation similarly.
Alternately, you could give a gift card to one of her favorite places or if you know a lot about her likes and or dislikes, you could get her some sort of gift. Although I'm not an in-home provider or a daycare worker, I am a teacher and if it helps I DEFINITELY love getting gift cards.