I think you're definitely right to point out the print run as a factor, Harlequin, and the press size too. I know some smaller presses can have sub 1k print runs. I do know one of the print runs from one of my books must have been deemed a failiure, as my publisher pulped some of the printed books - I only know this because they asked if I wanted to buy any before they did so!
I think it's totally reasonable for authors to ask and to be informed of print runs. I don't know mine for October 2021 release book, but I do know what the sell in to bookshops/supermarkets was, so if we go to print again I'll have some idea of what sold.
20k does seem very high to me for the average children's/YA. Less so if it includes ebooks too, but still high nonetheless (I've read many reports suggesting that children's/YA is still overwhelming dominated by physical books rather than ebooks). I'm often stunned by how low sales are of even very successful authors. In The Bookseller (UK trade magazine) there was a spread a while back on a successful children's/YA author who is definitely a career author, and it featured his top three books. The top book had sold sub 6k. Now, I'm thinking those are physical copies only, as it was done via BookScan, but even so, that felt low for an author who is definitely successful, and would as it stands probably have no problem having his publisher renew his contract. The second two books were around the 3k mark.
I'm bearing in mind here, though, that this is the UK market, which of course is much smaller than the US one, so any figures would need adjusting. It's funny you say that it's common for hardcovers of even midlist books in children and YA, Fuchsia, because that definitely is not the case here! Even pretty successful children's/YA authors go straight to paperback.I'd say hardback is reserved for the biggest releases. Lots of hardbacks in Adult, though.