I think you are handling this as well as can be expected, Susan.
To lie to you about his meds, take them even though he knew it would bother you, and then do it illegally without medical safety.. the first two are, in my opinion, inexcusable in a committed and serious relationship- lying is never the right way to go when it comes to something this serious and drastic- and the last one is simply irresponsible, as you said. Estrogen can be very dangerous if it's not regulated.

That said, I do wish it would have been possible for him to transition and everything while still holding on to you and your children. Kids can adapt to and accept more than we think; if you tell them the truth
without passing judgement or acting like their dad is doing something inherently wrong, then they should be fine with it, at least in time. [I'm not saying you would pass judgement or anything, I just wanted to mention it.] Kids are resilient. The real problem I see is that he lied.
Anyway, as for advice.. I don't really know if there's anything I can say, as I haven't had this experience.

But just.. try to make sure your kids understand that it's not their fault, that there's nothing wrong with it, that their dad still loves them and still wants to be in their lives [I assume], that it won't make them uncool at school, and that it doesn't mean
they have to be transgendered, too. If they feel mad or confused or upset, that's okay, let them feel it. Maybe see if he will let them ask any questions they have, if they want to know more? I wouldn't advise telling them the whole story about him lying at first and stuff, they don't need to know that.. it would just make things harder to take.
Finally.. is there any possibility of reconciliation? I mean.. can you two go to couples therapy and try to work it out? I know you hate liars.. but with something as inherent and powerful as the feeling of being transgendered, he may have felt as though he had no choice. It could be worth it to see if what you have is worth salvaging.