As Julia has said take Kenyan Shillings, I did not take any USD when I went, I always take a credit card with me when I travel, don't forget to inform the company that you are going abroad and going to Kenya. In the wood workshop that you visit this is the best place to buy the local wood carving etc, they did like English sterling money, I think they get a good exchange rate for it????...
It's a great tour to do I hope you have fun, if you need to know anything else just ask.
Cindy