Well, Shan-man beat me to the punch. The threads he mentioned should answer any general questions that you have. To answer your specific question about whether the experience might be too intense for a 6 year old and 2 1/2 year old, I will say no. Because you are in a private carriage, you can kind of control the fright factor. If no changes are made, the story is narrated on a tape recorder, and I would bet that a lot of it will go over the kids' heads anyway. Since you know what your kids might be able to tolerate, you can prepare them in advance as you see fit, and you can also ask your driver to take it easy, if that is what you would like to do.
Note that they tinkered with this experience the last two years, resulting in totally different experiences. It is possible that there will be more tinkering this year. If I had to bet, I would bet that whatever they do, there will be a Headless Horseman somewhere along the way.
Oh, and welcome to the Disboards!
TCD