"There is no greater agony than bearing an untold story inside you."- Maya Angelou
I'm going to look at this from a different perspective. But first; you raised these children and you know them better than anyone on here. Did you raise them to be open minded and accepting of difference? Sometimes when the chickens come home to roost, it's a good thing. You get eggs the next day. Sometimes, not so much. A lot depends on how you fed them.
This isn't really about whether they are comfortable or not. This is about you, and rightfully so. What do you want them to know about you? Are you ashamed of being a cross dresser? Do you want to explain yourself on your own terms? You have spent 40 years of your life living with this; are you sure you want it buried with you? Or would you rather leave behind you the knowledge that "this is who I was, quirks and all. This is my story."
I am not going to tell you to tell them now or not, or to leave a letter or not, and certainly, you should discuss it with your wife. But I urge you to understand that this is about you, your right to be who you are, and how you want your loose ends tied up and your story to be told. The universe, and your children can accommodate that.