I really don't think being transgendered or transsexual is genetic. I don't think it can be inherited. And I don't think it is learned behaviour from observation of gender roles of the parents. (Kids see too many other adults playing out gender roles for it to be only the parents.)
You might just create an opening for the kids to feel like they can speak out safely if they want to. I'm not sure how you would do that. You might talk one-on-one with each when you are both relaxed, perhaps mentioning how some people are born with the body of one gender and the mind/soul of the other - y'know, the whole TS thing. You have to present it in a non-judgemental way. I think you should present it casually, as part of other conversation rather than The Talk.
If you let them know that it is safe and OK to talk about such things, you may get some indication of what they are thinking. They may not talk about it right away, but if they know that they can then they will speak when they are ready.
This is just how I see it. I don't know if it is right. Good luck, dear.
- Kate