I haven't read your huge post to prevent spoilers on my part, but I'll chime in with my opinion on the matter.
I agree with you that having a darker tone for the sequels wouldn't sacrifice the appeal to kids. In fact, I'm pretty sure many of them would appreciate not being "babied" all the time with all of this child friendly content of sunshine, rainbows, and whatnot. In fact, many kid movies and cartoons back in the 90s and before had many adult elements in them, some very subtle, others more obvious. Some even had blood and alcohol in them, although brief. Of course, everyone's preferences are different and something to take into consideration, but now I feel as if they're too afraid to put these things in today's cartoons.