Many people suppress their transsexual feeling and because of societal pressures, attempt to live their life as their birth sex. After a certain amount of time, this can cause a real breakdown inside themselves andonce they finally accept what they are, they is no more denial. Some transsexuals know they are female and need to do something about it earlier in life (such as their teens) and some people have suppressed it until they are much older (50+). Many people may have thought they are alone and with the advent of the internet, have learned that there are others like the and there are treatment options.
Personally, I have know since I was little but have done a good job suppressing and hiding it from myself until recently. I married my wife and have kids as male and have constantly been searching for more meaning to my life and feeling empty. It wasn't until I recently stopped denying myself that I started to feel more purpose in my life. I am still married and for the time being, still plan on staying with my wife and being a parent to my kids. I try to take on the father role still, but as a female. They know they have 2 loving parents. When I told my children about myself, our youngest daughter (4) immediately took to seeing me as female and has called me Melissa ever since. our son (8) took a little time getting used to the idea, but now accepts me. My stepson (17) has had much greater difficulty adjusting and accepts who I am, but at the same time, goes to great lengths to try and avoid me.
Coinciding with my own experiences, there have been studies that show that the younger a child is when they are told, the greater chance they have of dealing with the news. Don't forget that children are exposed to all sorts of stories about transformation and magic (Beauty and the beast or the toad that turns into a prince for instance). The older a child is when told, the harder time they have dealing with it for a couple reasons:
1. You have gone X amount of years of hiding a secret from them.
2. Their ideas about how the world works becomes solidified the older they get and challenging one of the fundamental foundations of their beliefs can be much harder on them later.
Just remember, the father of your stepchildren was unlikely trying to cause any harm at the time when they were born and is probably trying to do things in a way that will be most beneficial to everyone. That is the most probable reason for only seeing the children a couple times a year.
You mentioned that she had made a "choice" to change her sex to match what she felt. Do realize that the alternate choices would have been death, misery or insanity, which would have been even harder on the children and would have nullified the father's purpose of living. To call it selfish is unjust when the alternatives are put into perspective. Unlike you, the father did not have a choice to remain living as the sex they did not identify with AND live a productive life.
Melissa