Let me start by saying it can't be done. Explaining why one crossdresses to someone who doesn't, is sort of like tryng to explain a particular sound to someone who has never been able to hear.
For years, I've been wanting to come out to my children, but try as I might, I could never come up with a way to explain it. First off, I know they'd be disapproving right from the get-go, and they'd ask me why I do it, and trying to make them understand and sympathise would be impossible because....well, just because.
My SO is very approving of my CDing, and she doesn't ask why I do it, so I didn't have to explain it to her, thank God. But I know my children, and they'd never approve. I couldn't bare to lose their love and respect, so I guess I'm just stuck.