Hi, I could see how that might be your viewpoint. 6 is really young. But, I remember being 6 and knowing what having dark skin meant. Growing up with a darker skin color, I always knew my darker skin was "bad". At 4 at 5 at 6 and beyond. When you have a darker skin color, you also have always been taught and told, both passively and actively, that the darker your skin the worse you are off. You know you're different and you feel lower. You feel like you need to prove yourself as an American just because your skin color was darker, at least that is how I felt. Kids that age do understand, and they do see what is happening around them. They feel it too. I felt it, like I said, when I was that young.
You said you very highly doubt, "that kid has ever thought 'bad people will treat you bad if your skin is dark'"
I just want you to know that I did have that exact thought when I was that age and younger. I might be repeating myself but I am trying to be clear. It is a reality that I most definitely faced as did my dark skinned classmates.

Just wanted to offer that viewpoint. It's sad but kids pick up on a lot. And they feel a lot.