honestly, I have yet to come out to my cousins kids, and I see them at least once a week. I talked to my cousin, and she said she will talk to them(the eldest is 7). children are usually more accepting maria, and of the two, the sister will probably be easier to talk to. I am the youngest sibling in my family so it was easier to come out to my family.....like that was smart......but since it sounds like your parents are accepting, then that means that the two children should be as well.....just expect to be asked a lot of questions, because the 12 year old at lest will know about the two binary genders and want to get answers.....im not saying to provide him with answers to all his questions, but you wont be able to hide it for long at any rate, might as well get it over with so they can accept you.....it will take time for them to wrap their minds around it.
I hope all goes well for you Maria.
Love,
Lilly Garcia