My problem with query letters is I think it's just easier to do for some types of plots or genres.
Yes there will be exceptions but in general, it's going to be easier to condense a 33k kid's book than a long literary tome about intersectionalism or whatever.
It's not an equal audition in that sense.
But a synopsis or cover letter - that's far more equal.
I'd argue it's not, actually, depending on the specific books involved, but in a general sense, I dispute your premise.
You might think it's easier to query a shorter book that you're thinking of as 'simpler' but it's really not. Think of it like putting together an outfit to impress someone. If you've got a big closet with a bunch of looks, and a big jewelry chest, you have a hard time choosing just what to pick, but there are options and ways to rearrange things that you don't have if your wardrobe is smaller. There, your options can be so limited it's hard to put together something impressive, or find the right thing without throwing on everything all at once. As well, a kids' book can have a basic theme, same as an adult book, but you don't generally want to come out and say it. You have to talk about the book and show the same things without mentioning the obvious, and without burning through too MUCH of the plot in a query.
Have you read the Queries for Well-Known Works, or whatever it's called, thread? There's a query in there for fucking Les Mis, which, I do believe, as I know who wrote it, clocks in under 300 words. There are several queries for some things, because they offer enough options to go several ways. There are queries for short things and long, incredibly complex stuff and simple. People have queries up for Shakespeare and Milne works. Anything can be queried. Anything.
Same as the famous one-sentence thing: sum up your novel in one concise sentence. If you can't, there's something wrong with either you or the book. People will protest this all the livelong day, but it's true.
