Most of these LucasArts graphical adventure games were written using a game engine known as SCUMM (https://en.wikipedia.org/wiki/SCUMM) that was able to target different platforms, which was ported to the Mac and so ran on 68k and later PowerPC Macs. There is now an emulator that can play SCUMM games on almost any platform known as ScummVM (http://scummvm.org/).
While I maxed out on the RAM (a whopping 512MB), as it's one of the first clamshells it's limited by a 300Mhz processor. So no OSX for me. I don't mind, it's on 9.2 right now, which is about the most stable as they come. And with my homemade ssd (CF card with IDE adapter) it's blazing fast!
I think the highest OS X version you can install on a clamshell is 10.4. I have it on one of mine (with memory maxed out to 576 Mb). As you say, it's not real fast, but ScummVM works, and so do C64 and GBA emulators.
I don't know how the parent is actually doing it, but many of these classic adventure games date back to a time when there were multiple competing architectures; the games were often written in some kind of bytecode, and getting them running on modern hardware just needs a new implementation of the VM, like ScummVM.
They are mostly pre-intel based games, as Apple switched to Intel in 2002. They are free for download through sites like macintoshgarden.org. Otherwise I use an emulator (Basilisk/SheepShaver) on my own PC/Mac.
How are you running these on a PowerPC/G3?