I know this is going to sound horrible to some people, but what 3rd grader identifies as nonbinary? IMHO sex and gender may as well be treated as the same thing until puberty, at least.
For some kids that's true. For a surprisingly large percentage though, gender identity can be known before they start school. Most will conform to their birth-assigned gender, so for those people, it isn't something they express clearly.
What social situations are kids put in that make them confront the question of their gender? Is it mostly like marketing and toys and stuff? Or more like family/social pressure to conform to "roles" (baseball v ballet)?
Just seems so odd for a kid to have to think about gender in general, I'm trying to imagine non-creepy situations where it would come up lol