Angela, you wouldn't be human if you didn't have doubts.
Transition is pretty much the most important decision of your entire life, so there is a lot to be considered. All of us are unique and our journeys are individual too.
You have young children, and any woman will always put them first. It's what we seem to be programmed to do, even if we are currently wearing a male body. Have you considered any sort of tentative time line for your transition and discussed the options with your wife?
If you have a supportive wife who is happy to have you dress more comfortably, and no doubt appreciates a a husband who really is capable of thinking and feeling like her, then you have found a jewel beyond price.
If your children are very young they would probably cope better than if they were early teenagers for instance. Young children see nothing wrong with things magically changing into something else in their stories.
You stated that you want to be a woman emotionally, relationally and physically. It seems to me that a lot of us eventually realise that the physical is actually the least important in many ways, and the others can be started at any time, without resort to the gatekeepers.
Transition is a slow process, so maybe if you can think of your marriage as being "Gender Queer" at least for a time, you may find a niche for a while . A lot of people pause during their journey, and some settle happily at some intermediate point along the way. Exploring all the nuances of being a woman is tough without a mentor, but having someone to share such things with is a dream for a lot of us. Take your time and get a proper education,so that if you do move further along the road, all that will be second nature to you, and you can concentrate on the next stage.
As for passing, I am 58, and really didn't think I passed all that well, until I came out to my Gender Studies class at Uni a couple of weeks ago. Not one of them had any inkling I was Trans, despite many of the questions I asked in Lectures being about Intersex and Trans issues. Not even the girl sitting next to me at the time.

A smile and a bit of confidence will do wonders.
Hope you can get some resolution to your situation.
Karen.