I understand your dilemma, I have an 11yr old son and a 9yr old girl. My approach was a softly softly approach, changing my modes of dress and letting my more comfortable feminine mannerisms filter into my behaviour in their presence just letting them get used to the change and ask questions if they felt the need, which inevitably they did, my initial answers were along the lines that clothing is a choice and I felt more comfortable in more androgynous or feminine clothing, then I gave them all the time in the world to process that.
Now here I am after the better part of 2 years of patience, I dress completely female full time and the kids are totally accepting of it, my son is not an idiot and I am sure he has pieced together much of what is going on but regardless I am still taking it easy with them.
I will never be their 'mum', that honour goes to my ex-wife, but dad is a word I can live with, it is a badge of honour and nothing to do with my gender. Dont get me wrong, I would love it if they ever start calling me mum but I understand that may be too much to ask.....
As for actually hitting them with the whammy that I will one day be female that conversation will come, at the right momnet, probably a moment of their choosing not mine, but regardless I think they already have an idea on some level and have probably already accepted more of this than I realise.
It is not a betrayal, you will always be their parent, you will always love them and you may be surprised to find that the whole thing is less of a problem for them, generations move on and society evolves, they didnt even bat an eyelid (barring a few giggles) when I introduced them to my new boyfriend 6 years ago and now they adore him.
They will always adore you, take strength and joy in their love and trust them.......
*************
Additional:
My fella reminded me that you can never truly know the inner workings of another persons mind, even your own kids and as I always say 'All you can do is give your kids the tools and help them to learn how to use them, then let them go'
With that in mind there is no guarenteed positive reaction BUT you know your boys better than most, and they know YOU better than most.
Good luck whatever you decide.
*************