A little kid at my painting camp pulled me over to the side and said “I don’t know how to ask this, but are you a boy or a girl?? I can’t tell.”
And I was thrilled because I had passed the “child pointing out your appearance means gender” test as well as being approachable enough that he could ask. So I sat down and told him I was neither and explained that growing up I was told I was a girl but that I’m not, but I’m not a boy either. And he just looked at me and said “so you’re just a person.” Swear to god I had the biggest smile on my face when I told him that’s exactly what I was.
For the rest of the day he just called me Izzy instead of Ms something or any gendered title and I got a hug at the end of camp.
So if you say gender is too confusing for children to understand, I have a first grader who would tell you otherwise.