I am at the very early stages of transition, so only my wife knows at the moment. Today, I was wearing some eyeshadow at home for the first time and the first time he saw me, my son immediately asked me why my eyebrows were purple...
I told him they were purple because I liked how they looked. My wife added that it's called eyeshadow and it's a kind of makeup.
That was that. (But children are kind of a special case...)