When my son was 4 he was devastated to learn that he would never get to grow a baby in his belly and feed it his own milk. He wanted to know if there was any way a man could grow a baby and if not, if there was a way a boy could grow up to be a woman. I was still very 'open minded' about trans, so told him some people who were born girls but live as transmen can grow babies and some people born as boys and live as transwomen but they can't grow babies. So either way, you have to be born a girl to grow a baby and he would not be able to do so. We talked about it on an off for a few days as he'd keep bringing it up. Once I explained that he could still be a parent as a man needs to supply an ingredient before the baby can start to grow which makes them a dad, he was satisfied.

Almost every single child who wants to be the opposite sex, wants that because they want to have or do something that they associate with the opposite sex because of what the adults around them teach them. Be it play with a Barbie, dress up like Elsa, do karate, dress up like Batman, etc. Children who grow up being told they can enjoy whatever they want (within practical reason) are very unlikely to feel a pressing desire to be the other sex. My son managed to want to do the only two things in life that he could absolutely never do because of his sex, pregnancy and breastfeeding, and was able to happily come to terms with the potential to be a father instead once we talked about it. That's all you have to do, let kids explore their likes and dislikes based on their personalities rather than their sex and talk to them rationally whenever they get confused or feel limited.