This varies greatly depending on the individual. Firstly, I want to point out that our group is respectful of religious customs and many of us continue to celebrate holidays based in faith because they involve family or cultural traditions. While some parents choose not to celebrate these holidays, many who do will change the focus from spiritual to seasonal. Quite a few of our modern religious holidays have at least some of their roots in far older earth-based spiritual traditions. Talking to our children about the evolution of belief leading to our modern holidays is something I have, personally, engaged in with my children. I also emphasize to them the respect of other traditions, including those that involve holiday figures like Santa.
Santa and the Easter Bunny are figures that are prominent in modern American mythos. They are important to many and we prefer an informal “don’t let the cat out of the bag” policy with regards to discussing them with the children of others.