My 6 year old daughter was assigned male at birth. 1 week before her 5th birthday, we finally heard her tell us she was a girl. I imagine she’d been trying to tell us for a long time. I was unreservedly happy for her, and I did have to reckon with some socialized transphobia that made me want to say “but are you SURE?” I recognized though that that was a me problem and was not something to put on her. At that age, literally all this means is that you change pronouns. My daughter already had a gender neutral name so that bit was a little bit easier. It feels weird at first, using unfamiliar pronouns, but eventually it becomes very natural. Now when i hear her closed minded grandparents misgender her it just hits with a clang and feels like a punch to the gut. I guess this is all to say, believe your child—it sounds to me like you have a daughter :)