I'm in the same boat with my children (6 and 4 years old), the older one is really good about trying new things, but the younger one not so much. I think what I hat most about the youngest ones eating habits is that he will declare food he has eaten hand over fist previously disgusting when served to him again and insist that he has never eaten it before. One you get him to actually take a bite he remember he likes it and will then eat it hand over fist again but it gets old fast.
For us fast food happens at very regular intervals, on the first Tuesday of the month and that is it. The Lego stores offer a free little mini model to build every month on the first Tuesday and we go and do that then I let them pick what we have for dinner. I don't understand children's love of cheap food stuffs like what is at McDonalds, White Castle, Taco Johns, or Panda Express, but it is a treat for them and the one day a month that they really look forward to is worth consuming some crappy food on my part.