Hi there,
I'm an Australian living in Kenya.
The Christmas period in Kenya is generally family time, so many areas will be quiet. I would recommend visiting the coast first, i.e. Mombasa, Lamu, Diani etc because that is where most of the culture and action will be. I would visit Nairobi after the New Year, because the streets are very quiet and not much happens in the capital city over christmas.
I hope that helps,
- Marianne.