My ex played a lot of games after I came out to my then 9 and 11 year old children. She took me to court to try to force the kids to call me Daddy even though they chose Maddy as their name for me. That was ignored by the Judge. She tried to force therapy for me and her, but I couldn't get a therapist to agree to take it on (even tried to bribe one with $500 cash to sit and listen). Therapists wouldn't cooperate because it was bogus, none of my ex's business and unethical for them to force me to listen to her abuse.
Bottom line, though - there are studies that prove that the damage from transitioning does not come from the loving parent who has transitioned, it comes from the hostile co-parent. Your ex needs to grow up and realize that this is a good time to teach your child about love, acceptance, tolerance whatever.
My ex continues to rail against me, mock me, deride me...as do her parents, right in front of our children. Every time our children hear the crap it strenthens their bond with me. You will be fine as long as you continue to be loving and attentive to your child's needs.