I never told my kids anything, and I have 5 kids (boy, boy, girl, boy, girl). My attire, mannerisms, etc., were always androgynous.
One day when my youngest girl was 13, she asked me: "since what age you knew you were a girl?" I answered: "pretty much since I can remember."
Oh! she said.... after a pause I asked her: "When did you learned about it?" She smiled and said: "Oh, I always knew your where a she-dad!"
I got curious, so one day when we all of us were having supper, I asked my oldest: "when did you knew I was a trans woman?" He said: [Well, I seen you wearing ladies undies all your life, so, one day I asked Mom why? She said: "well, because he is has always been confused about his gender," so I figure you were a transgender person].
The point of telling my story is that kids are pretty intuitive and have ways of figuring things out...
My advice is to love them, be yourself, reassure them that you will always will be their father, and provided them with direct answers ,short and appropriate to their age, to their questions...
Good luck !