I explained to my kids when at a similar age when they saw their first trans woman:

• some people are born man and love women, some are born women and love men. That is most people
• some other people are born men who love men or women who love women and that is fine. Many people hate this, but who is to tell you who you were born to love.
• some other people are born in the wrong body and that is also fine. These people may go to the doctor to help them get the right body. Many people also hate this, but there is really no issues with it as it is a personal choice.
• Whatever you are or love, things will get clearer when you grow up, but it may always be confusing and thinking about it is the best way to know who you are and love.

You can customize the last part and explain that you are like that, born a woman in the wrong body. That momy X knew they would have 2 mommies and yes, momy eve was born in the body of a man and we are lucky to have modern doctors that can help.