I don't think it's so cut-and-dry as all that.

The brutal offscreen execution of Oola produces screams but no mangled corpse. The brutal offscreen executions of Owen and Beru produce no screams but do produce mangled corpses. I'd say they're both pretty awful, but if I had to choose, ROTJ is slightly kid-friendlier.

Yoda dying a peaceful natural death compared to Obi-Wan's murder. No contest, ROTJ is kid-friendlier there, too.

Luke's torture is onscreen, yes, but I'd say Leia's offscreen torture trumps both Luke's in ROTJ and Han's in ESB. That interrogation droid was pretty much designed to scare the bejeezus out of kids.

And the roasted alive schtick? At the time, that was such a tired old trope kids were very familiar and it played for laughs even among the most sensitive kids. They saw "natives roasting explorers" on everything from Gilligan's Island to Scooby Doo. Nowadays that joke has been played out for decades, so someone born in, say, the late eighties might be totally unfamiliar with it, and actually be disturbed by what used to be a fairly pedestrian joke. But to people raised in the late seventies and early eighties, it was just a somewhat lame gag, like Chewie's Tarzan yell. Oh, the heroes are being roasted by the natives, just like a zillion other serial adventures before. Har, har.

That said, there's plenty of kid-unfriendly stuff in Jedi. The rape vibe likely went over most kids' heads, but it's still there. The Rancor and the Emperor were both flat-out terrifying (pigman or not), and Jabba's death was brutal, slow, and surprisingly onscreen. I don't agree ROTJ is the kiddie movie everyone makes it out to be, but it's hardly the dark one of the trilogy.

And, back on-topic, I don't think the relative kid-friendliness of one movie over another, which is arguable, is really even tangentially related to R2's cheerier personality in ROTJ, which I find to be pretty much just the way it is.