How to explain to my four year old that his step-parent is transitioning?

[Selenakyle]

My partner is in the early stages of transitioning.  I have a nearly five year old year old son and I'm still trying to figure out the least confusing way of telling him that the person he has always known as a man is actually a woman. Trans folks with young kids, how did you handle this? 

[Asche]

I don't have young children, but since no one else has responded, I'll
put my USD 0.02 in.

From everything I've heard, young children have a much easier
time with it than older people.  They're used to the idea that
the world has lots of mysterious things -- Santa Claus,
the Easter Bunny, where babies come from, etc.  The idea
that someone who everybody thought was a man was really
a woman isn't any more mysterious.

Typical explanations are, "we thought she was a man, but it turns
out they made a mistake and she is really a woman."  Or something
like that.

Jazz Jennings describes how when she was little, her
friends didn't have any problem with the fact that she,
a girl, happened to have a penis.  Weirder things had
happened, after all.  In every other respect (and in all
the respects that count at that age), she was a girl.
It was always the adults that had a problem.  (It
became more of an issue for other kids when she was older.)

[DawnOday]

Kids are born with no preconceptions. Every bad thought is instigated by someone older who had learned their prejudice from someone older. Kids understand kindness, attention, positive attitude. If your four year old looks up to you. you will have more influence on their developing opinion than anything else. If they see you treat your spouse with kindness they will follow suit. By the time he starts accepting other kids opinions you will have prepared him.

[arice]

I would go with something along the lines of: some people are born with a body that looks like a boy but inside they feel like a girl/lady. Then tell them that's what is happening with your partner.
Kids generally are good with that kind of thing.

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