Plenty of children read way above their reading level and encounter adult themes. I certainly was one of those. But there's something that feels off about novels like Piers Anthony wrote that include not just adult themes, but adult themes that specifically target children.
In retrospect, and speaking of the non-Xanth works (mostly because I never wanted to read those), I think it was because it seemed like it was more adult than a lot of fantasy (or science fiction), while also ... kinda aimed at the teens.
Which is flattering at the time, and super not-so-fine in retrospect. Kind of like when you're an adult, and you suddenly come across that Benny Mardones video on youtube for Into the Night and you're all like, "Wait a minute ... this was a hit song? What were we thinking?"
No, not impossible, and as
@Sacrosanct mentioned, they originally had the idea of ascending armor class on the table for 2e. Whether they would've just developed more attack matrices for that, used a simple to-hit bonus, or stuck with a version of Thac0 as to hit armor class zero being what you need to roll to hit the worst possible AC, not the exact middle, who knows?
I don't think it was inherently impossible to make the jump from attack matrices straight to BAB. I do think it would have been impossible to make that jump in 2E, however, because 2E was trying to stay more-or-less backwards compatible. Going to BAB means flipping the direction of armor class, which in turn means all monster and NPC statblocks using the old system are invalidated.
I've since re-read a bunch of books that I first read in the 10-15 age range, and I cannot imagine my parents had any idea what was actually in them, just figured "oh, it's fantasy (and part of me suspects, on sale) and little Ralif likes that so we'll get that for him."
I think a part of it also was that it much of it was aimed at ~14 year old boys and had a bunch of jokes in them. People looked at that and said, roughly, 'ah, this guy is writing acceptable-transgression material for the pubescent set. Good job. Let them think they are rebelling with their fart jokes and panty references,' and the more egregious bits got attention-drowned-out. Kinda like later when the someone noticed some ching-chong-china level stereotyping in some Captain Underpants novels (hiding in plain sight under the, well, underpants jokes for 8 year olds).
If I recall correctly, the CD that came with the 3e PHB had conversion rules for going from descending to ascending AC, amongst other things. Old-School Essentials actually lists both AC systems in it, with each monster stat giving their AC in both formats. People could've absolutely managed the changeover then. In 1989, the RPG scene was awash in far more complicated rules systems than that. But one wonders what the response would've been if you had half-orcs, assassins, demons, devils, and descending AC all gone in the initial release of 2e?
I think they could have done it simply by having a disclaimer at the front about converting AC of old material from 20 or the like. If they trust someone to use thAC0s, they must trust them to do that kind of simple math, right? They simply didn't have the appetite to do so, as pressure from on high was to maximize the backwards compatibility. It may or may not have been the right decision to retain the most 1e players (I think other factors, such as how well TSR was being run at the time, undoubtedly dwarfed this one), but I have faith that if they'd done it, people would have adapted.