Hello Alison, have you heard of a tv show called 'Transparent'? It's about a person who realises in middle age that they're trans, and goes through the whole story, including what happens to the wife and children and how they all relate to each other. The characters in the show, including the children, are all adults which perhaps is a bit different to your situation. I found it really moving, especially as over time the main character (Maura) has flashbacks where she realises she's had these feelings from a very young age.
It sounds like you're at a real crossroads here, your real identity is starting to become clearer but it may not be possible for you to live that identity and have the exact life you have now. Keeping the family together is really important for you too. When you say you're scared of hurting your girls, what are you worried might happen? As Nat and Pamela have said, there are a number of different ways this could pan out. What would you like to happen in an ideal world?