Since World of Warcraft was mentioned, perhaps Morrowind and Skyrim would work despite the violence. Also, there's the Myst games I suppose.

https://www.amazon.co.uk/Wild-Earth-Africa-PC-CD/dp/B000MFQZ0W

I did once ask the developer to release on GoG and he said he'd look into it, but it never appeared here.

Well, it's not on PC, but if you can, try "Pokemon Snap" on the N64 :)
"Ecco the Dolphin" was a real pleasure for me to play when I was young (the first one). It's always nice to swim and discover the sea, even if there are enemies.
May be not an exploration game, but a very nice educational/adventure game : "Zoombinis" (PC 1996) (it begins with a very low level of difficulty, and rise each time you succeed, and become interesting even for a grown up).

"Spore" can be interesting, when you design your creature in 3D and discover the world.
"Shelter" can be nice too for the style and discovery.
"Octodad" isn't really exploration, but may be it can be interesting to take a look.
And "Stardew Valley" of course :)

And one that I only know through what I've read and saw : "The Witness". Exploration and logical puzzles.

I don't know that "exploration" would be the right word for them, but how about the <i>Scribblenauts</i> games? probably great for playing with a little kid. Never played one myself, but I wish <i>Scribblenauts Unlimited</i> would come here.

Or maybe it's time to get DosBox working and try some adventures or games like Dizzy.
Audiosurf is an interesting game which content is based on music you put in.
And how about something story driven like made-into-game gamebook Steve Jackson's Sorcery! series?
For playstation there are things like Journey and Flower.
Then you can have some old classics like Pac-man, tetris, asteroid, pong or other simple titles that are both fun to play and watch.

Lego Worlds.

When I was 5 or 6 my dad played Myst while I watched. It's possible that Myst might not be as captivating for a child of this generation due to the march of technology, but its successor Obduction may fit the bill.

Some great suggestions here-- thanks so much!