I haven't done my sums, but I think that the proportion of characters among the children whose names indicate they are probably not white is about what you'd expect in a country where fewer than one in ten of the population are non-white.

Since most of the readers of the books would probably be Muggles, I'd think that providing examples of Muggles being represented positively would be superfluous. With the exception of Harry's foster family all references to Muggles do seem to be sympathetic, except in the mouths of characters we are meant to dislike.

As for class, there is little indication of this so far as most characters are concerned. The implication is that wizarding skills are a lot more significant.
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I'm looking forward to seeing the film when the kids have gone back to school after half term. I'd dread seeing it in a cinema crammed with children...

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If I wanted to point a child to a book that would be a natural progression from Harry Potter, I'd suggest Ursula LeGuin's Wizard of Earthsea.