Tigger, I'm currently going through something similar with my SO, the difference being that she's a lifelong dyke -- which is how I identified until I realized I just couldn't do it any more. Also, there are no kids in the picture so I can't speak to that.
It's hard. She's very supportive, but at the same time, she doesn't really get it, and it's very difficult for her, especially now that some effects of T are starting to show (I'm about three months in). The more she's able to think of me as a guy, the less she's able to think of me as her partner. But the ironic thing is that in spite of that, this is bringing us closer together; partly because I'm so much less depressed, more able to open up and be vulnerable emotionally, and generally nicer to be around.
I think the key to this is that I've chosen to be very open with her, as honest as I know how to be about what's going on. Making oneself vulnerable in this way is one of the hardest things there is, because the fear of losing this most important person is so strong. But if you really respect and love someone, it's the only way to go. We tend say "Oh, I don't want to hurt them," or "I have to protect them," but I'm pretty sure, at least for me, that what's really going on when I say that is that I'm protecting myself, not the other person -- from fear of rejection, fear of being hurt -- it's about me. So I've learned the hard way that it's a selfish thing to do. My partner is her own person, and she gets to decide how she feels about things; I can't control that, and it's disrespectful to try.
Our relationship is changing... but it's also getting stronger, and I'm working pretty hard at letting go of needing to control the outcome.