How to tell a 9 y/o that I'm trans?

Hey all, I need advice.

So I came out to my Aunt, chosen family, and she's supportive of me.

I start T tomorrow (yay!) So it should take a bit before any real changes happen.

Me and her are trying to brainstorm how we'll explain this to my little cousin, who is a 9 year old girl. She's known me as a girl this whole time, a few years, but she's old enough to have thoughts and opinions and processing.

Her mom doesn't want her to get the wrong ideas (completely understandable) as she's still very little.

What do you think would be the easiest way to explain this to her, in a way that is digestible and won't affect her way of thinking much?

PLEASE be respectful. I feel like i shouldn't have to say it but I know the internet. Obviously there is nothing wrong with kids knowing trans folk exist, HENCE why we are telling her. But it is completely understandable and within fair parameters to not want to confuse a little one on identity. COMING FROM SAID TRANS PERSON. Thank you.