Both my kids (3 and 5 but now at school) have said stuff like “Today we did x with Greg. He’s not exactly a boy and not exactly a girl.” Then they get on with their day. To them, it’s just another person that’s a bit different to them.
I live in a pretty liberal European city, yet I haven't met any non-binary person. I know exactly one trans person, and only remotely (she lives in US). I guess to me, this topic seems "advanced", perhaps "irrelevant" in a way for such a small kid. There are many other things she needs to learn about, which she will commonly experience in the real world.
Being nonbinary is kinda unusual, but it's also probably something that's super easy to be closeted about, if you don't know how people will react.
I mean in the 1980s I thought all 900 kids in my high school were straight, and that being gay was super uncommon.
Future "not evenly distributed" and all that.