I have my 5 yr old daughter set up with an iMac (from back in the day)... since she was 3-1/2 and had installed 8.6 on it and I have to say... it is still chugging along like a champ.... through countless freezes in games (not due to the system but the game itself, where an item gets literally stuck where it is)
And the fix?? Too simple.. just turn-off the multi switch, turn it back on... turn the machine on and... viola!!

It still has the fastest boot and shutdown times I have ever seen, and the original desktop wallpapers from then are still some of the prettiest I have seen even to this day.. (She loves the pink jellies

)
Granted, it's an old OS but, I am still able to find new children's-educational software (as well as other stuff) to purchase for it. And it has not ever mattered how many programs I install on this machine, it never seems to slow down.
I have newer Mac OSes... the latest being 10.2 Jaguar (which I think was Macs best ever... ever compared to anything newer) But, 8.6 is just a perfect match for my lil Princess..... Cool it has the slot-drive CD/DVD as well too so she watches movies on it sometimes and the picture is beautiful
So, my cousin was over, who has his boys on WIndows XP and he was amazed how quickly I had her back up after a game froze... and the fact that it has happend numerous times (I told him) and there is never any lasting effects to the OS.... where his boy's machines are an absolute mess?
I'm wondering, anyone else here have or remember any old OSes like this which remain so great?