Hi GypsySoul,
Moral support from me too (my partner is MtF)!
I think it might be time to tell the kids soon (in an age appropriate way). Otherwise they'll be spending lots of time in dread that it's a divorce or something (which they may have witnessed in their friends' houses). You don't have to go ALL into the details.
Today I told a 10 year old (who had already heard stuff but was confused about it). Basically I said,
"When [Name] was a baby, some things happened that were different from the usual. She didn't get the usual hormones, and so when she was born it wasn't wasn't clear what her gender was. The doctor decided she was a boy. But when she grew up she decided to be a girl."
This seemed to take care of it for him. You could say, "Daddy thinks those doctors made a mistake, so here's what we're going to do..." and then talk about how much you both will always love them. (Sorry, I don't mean to lecture, I only got as far as step-mom myself.) In my experience if YOU are calm about it, they will take their tone from you, maybe have to "work out" some things...I think the big news will actually reassure them that there's a reason for the tension.
And amen to the opportunity to do, ah, EDUCATION in your small town!
Shannon and Jeneva, you two are so inspiring! Thank you AS ALWAYS.
Hugs,
Maddie