I have two small children who were 2 and 4 when I came out... They're 5 and 7 now.
Since then, I've learned that the simplest explanation is best when it comes to pre-teens... "I was born a boy but always felt that I should have been a girl
so I went to the doctor and she helped me become one."
I know that's a huge oversimplification of transsexuality but it works. Even when my daughter has introduced me to friends, she's said, "This is Jess...
She used to be a boy," they'd give me a look, shrug their shoulders and were off to play. Otherwise, you're a novelty that they'll talk about when you're not
looking but then just move on... Funny thing, though, how easily kids seem to understand; I offered a very brief and simple description
about the reason I would be in Thailand for a month. My daughter explained the whole thing to her therapist...in detail

.... she had just figured it out on her own.
Teens, however, will try to figure you out.. I have a niece, for example, that stares at me with this puzzled look, constantly whenever I'm around..
She calls me Aunt, which is really cool, but it's like she can't reconcile who I was with who I am now. I offer as much explanation as is owed someone
and is very specifically tailored to the amount of respect they show me. If your cousins are all over 12, I would offer to sit down with them and be absolutely
honest and open about things... answer their questions tactfully but honestly. Transsexuality isn't an "adult" subject but they'll see it as such if you try to
dance around the reality.