Well it's not just white children that comment. My children never noticed anyone's colour for ages. I think that children just know people and then one day they see someone they don't know and it occurs to them that there are differences. Here are a few of the examples of that within our family and the friends we have:
DS1 on seeing an African's children's choir at school came home and said
"Mummy all of the African children have gone home now"
(Me) "That's nice dear"
On seeing black people on the street
(DS1) "He's from Africa. He hasn't gone home. She's from Africa"
(Me) "And what about Mummy darling?"
(DS1) "You're Mummy" - so clearly my black skin hadn't occurred to him.
E (a friend's DD) on coming home from her grandparents (where they pass racist comments)... "Look mummy there's a c**n"
My friend was mortified and said "That's not a nice thing to say. What about Mars? She's black"
E replied "Don't be silly mummy, she's Mars!!!" - again my black skin didn't occur to her.
DD2 turned around one day and said
"Poor Daddy, I'm brown, DS1's brown, Mummy's brown. Poor Daddy is just white!"
I picked up a friend's daughter from nursery the other day. She looked and looked at me (she's known me since birth 3.5yrs ago).
"Why do you look like that?"
(Me) "Like what?"
(F) "Like that! All over!"
(Me) "God loves colour and made us all different colours. It would be boring if we were all grey or the same colour. Your mummy and daddy are white, so you are white. My mummy and daddy are black, so I am black. I am black, DH is white and so my children are a mixture and look a lovely caramel brown"
(F)"Oh I see!"
She had recently come back from a trip to Africa and I think that that made her notice.
What I'm getting at is that kids start to notice differences and being kids just come out and say it. I doubt the guy in the queue gave it another thought! I certainly wouldn't. In fact I kinda chuckle inside as I watch people squirm and try to make me feel better about something that didn't upset me in the first place (but then my humour is wicked and bad!)