DS & I were in the PO a few months ago. DD1's friends father walked in. Dark skin & dreds half way down his back.
DS: 'mum, has he got a knife?'
Me: 'no, why?'
DS: 'he looks like a naughty boy...'

He refers to people he doesn't know by the colour of their clothes... So its lovely when he says to me 'mum, whats that black boys name?' coz the poor guy is wearing a black tshirt, lol.
My kids don't really notice skin colours & with the comments about bigger people etc, I just tell them 'shh, its not nice to say things like that'. Its worked so far.

Except when I was pg with DD2 & DD1 pointed out a bloke with a big belly & said 'look poppy, that man's having a baby too!!'

We have a close family friend, who's in a wheel chair after a bike accident. He loves kids because they are so honest & up front. They just ask & don't get uncomfortable, or ignore the issue.