I have no problem at all believing a kid could just randomly work this comment into a conversation and have it be totally innocent.
I don’t like kids. I don’t spend much time around kids. So, I don’t have much experience with them.
That being said, I can not remember ever meeting a kid between the ages of 3 and 12 who did NOT make a statement like that to me, completely out of the blue.
For instance, yesterday I was having a normal conversation with my niece (she’s almost 4). Out of nowhere, she said, “you’re much bigger than I am.” I restrained myself from saying “duh” and the conversation continued.
My nephew told me (when he was 5) that I was fat (thanks you little shit). He told me again, last week, that I am much bigger than his mommy (she is 5’3" and weighs less than 100lbs). At least he’s gotten more tactful in the last 4 years.
The girls I used to babysit (ages 8 and 10), frequently commented on my skin color (very pale), my eye color (blue), my hair color (coppery), my boobs (big), my stomach (big but not as big as it is now), my height (short). These comments always came from nowhere and had no relevance in the actual conversation. The girls noticed these things about me because I was unusual to them. Their friends were mostly Jewish, mostly brunette, mostly brown-eyed, mostly olive skinned. The girls saw the differences in me and pointed them out. They were pointing out facts. They were not trying to hurt me by telling me that I was fat and pale and that I had a strange hair color. They were just being kids and making observations. I could have taken offense and told them it was impolite to point out things like that. Instead, I took the opportunity to teach them a little about genetics. We made up little charts with the known traits of all our living family members. They enjoyed the exercise. By expanding on their families and talking to their parents, they were actually able to get rid of their own stereotypical belief that all Jews had brown hair and eyes (they acutally had 1 blond and 1 redhead in their family).
Just recently at my new job, my sister was talking to two coworkers on third shift. She told the supervisor that I would be coming to third shift in a few weeks but was training on first. The supervisor had met quite a few new female employees recently and asked which one I was. The mechanic described me as, the short girl with long red hair and glasses. Oh, she’s also bigger than Mary but in all the right ways. If anything, I think my twig sister should have taken offense to that statement. I thought it was funny.
Saying, the white girl wouldn’t have made much of a point since most of the people on first shift are white. He could have described me as the girl who’s not a hick and has all her teeth. That would have been much more descriptive but probably never occured to him. 
Anyway, kids learn by making observations and by receiving feedback on those observations. I used to make that type of observation when I was a kid but since I was shy, I usually made them to my mother only. She would then elaborate on my observations. When I told her that her coworker, Anna, had a funny accent, she explained that Anna was from Puerto Rico and grew up speaking Spanish. We then read a little about PR so I would understand what and where PR was.
The only time I got in trouble for making an observation was the only time I made it to the observee’s face. I had been playing with the girl’s hair and when I stopped, she asked why. I said that her hair was making my hands greasy. I got grounded. It’s been almost 20 years since that happened and I’m still irritated. All the adults who heard insisted that I was making fun of the fact that she was black. Until they mentioned it, I never even noticed her color. All I knew was that she had unusual hair and it was fun to play with. Then I noticed that her hair was greasy.
You know what? Her hair was greasy. And I don’t think it had anything to do with her race. However, I learned that people do not like it when you point out that their hair is greasy. I have never said it to someone’s face again so I learned my lesson. But it was not a racially motivated statement. People need to stop being so damn sensitive all the time. Of course, the greasy hair incident happened around 1988 so it’s not like this PC bullshit is a new thing.