So you'd be content to call my child "they/them"? If children are truly gender neutral, and allowing feminine gender expression isn't okay, surely allowing masculine expression isn't either.

Having a child who, based on their behaviour, might be trans becomes very quickly about how you respond. I have a boy who consistently chooses girlish clothes, games, and movies ever since we allowed him that choice starting at two years old. The appropriate response is to let him explore whatever gender expression he wants: let him wear the dresses he likes, let him like Frozen and My Little Pony, let him use barrettes and hair bows when he wants to. If he's cisgender, he'll feel like he can explore without his mom pitching a fit, if he's transgender he'll feel affirmed and have less mental health consequences.

And for taking that approach and allowing him to do what he likes I've been called a child abuser, a monster, and told that I should lose my kid. My mother who believes the same as you do, that toddlers are toddlers and can't know, keeps scheduling surprise haircuts and sending me tips on how I can convince him to dress more like a boy. "They can't know" often translates into enforcing the gender expectations of their birth sex, which isn't neutral at all.

Being open to the gender expression of toddlers is important to their happiness, but I think more important is being willing to be in their corner. Maybe he's cisgender, maybe he's transgender. I just can't know until he gets older. But there's nothing wrong with acknowledging that there's a possibility and that his current interests are one of many indicators. And being ready to shelter him from relatives who want to wrestle the nail polish away from him.