That's the thing that just wooshes right over these people's heads. Kids are kids; they don't know much about the world and so they expect the adults around them to know what they're talking about. It's not that the kid suddenly "understands" gender identity or non-binary people. It's that the kid is taking what you say at face value because you are a trusted adult and they don't have any reason to believe you would just make shit up.

If I had an 8-year-old kid and told them that bacon was made out of cats, they'd probably believe me. That doesn't mean that the kid is stupid and it doesn't mean that bacon is therefore made of cats. It just means that I lied to my kid and they believed me because they trusted me.

The problem is that if every adult around them is going around saying that non-binary is actually a thing, they're just going to grow up either (a) rigidly enforcing gender norms that "non-binary" is supposedly breaking down, or (b) really confused into thinking they're neither a girl nor a boy because they like purple instead of blue or pink.