but still not right

"And jolly well it might"" the harshly-named principal antagonist bellowed quaintly, as people maybe did in the author's youth. "You spuntfuttering numpthrapper!" She stood up, stepping into the light.
There was a brief pause, as the reader swiftly scanned the following paragraph. It was another where the physical characteristics of the character were described, rather unflatteringly, in ways the male characters seldom were.
"Then she said something else", he continued, "but the following dialogue also bears chiefly upon the physical description, so wouldn't make sense, following the excision of the contextualising setup." The conversation, excluding the diversion into appearance, continued proper after a few more lines. "It jolly well might and it jolly well will!" She rattled her claws excitedly. "In England, it would not be at all unusual!" He had long since stopped wondering whether to try adapting all geographic references to be less specific. "English numpthrappers would be especially easy, particularly for me!"
Fortunately, the occasional accompanying illustrations do not dwell so heavily on the occasional dips into appearance-based classification. There was even passage in The Twits specifically about the unimportance of features when set against intent and expression. Though it still drew attention to the features by mentioning them.
"Sometimes passages will be unavoidable!" cried the unflatteringly-described one evilly, "when vital narrative is too closely intertwined with casual sexism or the other tropes of generations past!" She went to the bingo, stayed at home cooking and tried to attract a rich man to marry. "It'll be unavoidable! Some of it will get through, you invaltrinated wingletumpter!".
But, unfortunately, it can be done, though at some point the child will read for himself and spot all the bits he doesn't recall from when they were read aloud. At that point it'll be more appropriate to explain that the books are from another age, albeit a relatively recent one. Interjecting explanations about the different attitudes prevalent in the past rather tends to break narrative flow receipt at this age. I'm slightly worried about how it went unnoticed when I was small. Precisely because it did, I'd prefer it to be noted this time.