You mean Skylar & Plux: Adventure on Clover Island?
If so, it's already delisted sadly :'(

Sorry, yes. That's the one.
And Ah, shame.

Okay, so it has occurred to me that both Deus Ex 1 and Fallout 1 have violence settings that turn off the blood and gore (and language with FO). Anyone recommend them as "your kid is smart, they can figure it out" kind of challenges?

I think turning off blood and gore (or coloring it green) to make a game seem less violent and more child-friendly is a very silly concept. A few colored pixels are not going to mess up children, it's more about the whole ideas behind and in the games. You'll still go on a killing spree but eveyrthing looks nice and clean? That doesn't sound any less disturbing to me as a concept, on the contrary ... XD

I'd suggest Torin's Passage and Samorost.
also, second Cave Story. it's fantastic.

not on GOG but there are those old Dizzy games.

Anything they want. Why would I care?

I guess the difference between having a characters brains splattered across the screen and them appearing to go to sleep can't be differentiated by the pool (or lack there) of red pixels encircling their rigid torso ?...

A lot of kids are gullible (age dependant, obviously), as it comes from inexperience. Oh, and AK 47 paint or pie guns (think: Bugsy Malone) and plastic blades are a thing, apparently... ;-)

Turtles don't drop coins if you jump step on them.

You seem suspiciously sure of that.

Psychonauts

the answer is always Psychonauts


I would also recomend Opus Magnum. It may seem a little hard, but it is so free form that anyone can create their own solutions, and it is a good game to learn computing and thinking skills

I second Terraria and the Lego games. Awesome games for young and old and can even be played together for more fun.

Zniw Adventure - it's point & click adventure game, has very universal content (should be fun to play for both adults and children), not too hard, IMO great title. (Plus, by some reason, many children like dinosaurs)
https://www.gog.com/en/game/zniw_adventure

Crystal Caves HD - platform game, very fun and "universal" content, violence is minimalistic and directed against things like robots, snakes etc (shooting with the laser, without blood)
https://www.gog.com/en/game/crystal_caves_hd

Thrillville: Off the rails - basically a collection of minigames with very simplistic theme park manager, (I guess that children will appreciate the fact,, that your alter ego in game's world is either small kid or teenager), even if there is some kind of violence in some mini-games, it's cartoonish and not drastic, e.g. cartoon tanks shooting to each other).
https://www.gog.com/en/game/thrillville_off_the_rails

Not sure they've been mentioned already (looking at endless game names made my eyes go fuzzy), but here's some on GOG that came to mind:

Jazz Jackrabbit

All three of the Creatures games (my own child played and enjoyed these as a pre-teen back then...):

2001 Creatures Village
2004 Creatures: The Albian Years
2004 Creatures Exodus