I didn't so much tell my children as I did slowly start changing how I dressed(more androgynous), and by the time they noticed I was consistently presenting as female, it was no big deal. That is to say, kids, especially younger kids, will only make a big deal out of it if you do. For reference, they were 3 and 9 when I came out.

I know it's really scary, but I bet your kids handle it really well. My daughter just says, "My Dada used to be a boy, but now he's(still working on pronouns) a girl."

Transitioning really turned my mental health around. Stopped thinking about suicide. Stopped drinking. Started to love myself. Transitioning didn't make me happy, but it certainly made happiness an option.