Thank you for touching on this topic! This discussion has gotten so far from the truth that it is causing so much more harm than good. You are right – X and Y chromosomes. You change the physical aspects of a human, but their fundamental design is determined in the womb.
Children are highly impressionable and learn from what they hear and see. Their brains aren’t even fully developed until well into their 20’s. I’ve known of cases where young children where part of hateful custody battles and the mother accused the father of sexual abuse and the child went to a lot of counseling (ordered through the court) and they were actually “brainwashed” into thinking they had been touched when, in fact, they never were. My own sister was conditioned to think something that absolutely didn’t happen. I’ve also known of many young children who decided they were a dog or some other animal and ran around acting like it for a period of time. As a parent in this situation, you would most definitely allow the child to have his/her fun but you would also let the child know that is just play because he/she is actually human and not a dog. What is the difference between that and your little boy wanting to play dress up with his sisters?
So to allow a child of this age to “make up their own mind on their gender” is just wrong! Like you said, you wouldn’t let them balance your checkbook! I know this is a sensitive topic and most discussions turn into hateful interactions.