I don't think it is harmless, either, though I do believe it was sung without real malice - it was thoughtless rather than harmless. It is certainly outrageous, and I have been telling everyone in real life about it, and watching their jaws drop! Happily, people who know me know I am not prone to making up stupid stories, and I am believed
I live 300 mls away from dd, sil and dgs, so we don't see them often - returning to the bookshop for the singalong session (it's a regular fixture) may well be on the cards in a couple of months, but I'm not in a position to be certain of doing that. DD may want me to take him swimming or something, or if it's a nice day, go somewhere outdoors.
If I do go back, I will report on what happens....and if the verse is sung again, I def will say something.