You can open a project in C++ to create the structure of a custom game. You put one child to work on blender or pixelart software, and another child to compile variables and functions. If you have more than two children, you can also consider assigning a third to animation and a fourth to creating music and sounds.

Not sure how old your kids are, but mine are 7 and 11 and I would definitely NOT play BG3 with them. If it were a movie, I don't think it would even get an R rating given all the sex and gore.

(To be clear, not judging the game. I quite enjoy it. Just don't find it appropriate for my little ones. Caveat emptor.)

FWIW, I had a lot of fun playing thru Portal with my son. No official co-op mode in the first one, but the second one has it and it's pretty old at this point so should run fine on older machines.

Minecraft? A server is extremely easy and cheap to setup

Minecraft, roblox, old school runescape, terraira, magicka and magicka 2, relm of the mad God, Diablo, path of exile

There are a ton of couch co-op games! Just not AAA

• Ultimate chicken horse (great for kids!)
• It takes Two
• Overcooked 1 & 2 (difficult for kids)
• jackbox games for parties
• Super Bunny man
• cuphead
• human fall flat
• lovers in a dangerous spacetime
• magika
• all Lego games (they have everything for kids interests)
• moving out 2

Neverwinter Nights has coop. It even has a dungeon master mode where one controls the game for the other players.

BG3 might also not be suitable for kids anyways. It's full of gruesome imagery. Like at the beginning you can pull a brain out of a more or less living person. And later on it gets much worse with mutilated bodies everywhere.

Another game to play would be Diablo 2. Also gruesome imagery but more removed from the players. If your machines can't handle Diablo 2 Resurrected, they should at least be able to run the original. And for the original you would also only need one copy between all players. But keep in mind that D2 is more action oriented.

You didn't mention ages, but my kids liked the LEGO games. We played through Harry Potter, LEGO City Undercover, Marvel Superheroes, and Jurassic World, and there are a few more we haven't played yet. They'll run on a potato and they're really fun.

They aren't RPGs though, but for my kids, that didn't matter. They could role play just fine with the characters they could pick between.

Other than that, there are some others that may fit the bill, but they often have mature themes, like Divinity Original Sin and Wasteland, but hopefully those give you enough to find something that'll work.

I personally play a lot of single player games with my kids watching. We're playing through Zelda games right now, but we've played a number of games on PC as well. I'll sometimes let them control things for a bit if they want, but generally I'm the one actually playing.

Since you mention Baldur's Gate 3 you could try Divinity Original Sin and DOS2, DOS has 2 player coop by default and with mods you can play 4 player, DOS2 has 4 player coop out of the box.

Although depending on how old your kids are it might be difficult to play.

Someone else already mentioned the lego games and I can't 100% recommend those as well. Lego Star Wars complete saga, lego star wars clone wars, lego indiana jones are the ones I've played and can recommend all of them.

Although there's only 2 player coop.

SuperTuxKart

A bit controversial, but... any game of chess. They'll thank you later.

Some of the best family friendly co-op games I've played:
Stardew Valley
Valheim
Don't Starve Together

I think the obvious choice if you want something similar to BG3 would be the other RPGs from Larian. DOS:EE and DOS2:DE are both coop, both round based RPGs and can both be run on Linux. While DOS2 is definitely the better one of the two (and in fact the reason why Larian was contracted for BG3) it could be hard for the AM3, depending on the exact type. But even the first game is just a very good roleplaying game. From all RPGs in the Iso perspective I've played, I liked Larian's the most.

There are of course also the older Baldurs Gate games. Both are coop (if I remember directly, I'm sure at least about the first one), again both can be run on Linux and on old systems, since they are old games. Which is also there main disadvantage. If you're used to better graphics, gameplay, games where you don't just miss all the time and voice actors, these games just can't really stand a chance to the Larian produced ones.

Neverwinter nights should be able to be run in coop, but I don't know if it runs on Linux. Otherwise it has pretty much all the disadvantages of the old Baldurs gate games without it being Baldurs Gate. Pick any aspect of the game and compare it to Baldurs gate and it'll loose. What it has going for it, is the pure amount of fan made campaigns for it. So if you and your kids like playing it together, you won't need to stop any time soon.

The only other "real" RPG coop campaign in my mind would be solasta, but I guess this would be too much for the budget PCs.

If you go away from the pure RPGs, there are some more options. Borderlands 2 can be played completely coop, has a pretty timeless graphic which is still easy for most systems and can be run on Linux. But while it has one of, if not the single best villain in gaming history, it's technically a loot shooter and not an RPG. Even if one of the DLCs literally is a DnD campaign of the characters.

Relaxing the RPG theme further, we see some more options looking at indie games. Games like Stardew Valley or Terraria are both simple, can be seen as RPGs by some and come with multiplayer. Both are very famous and not without reason. If the other games don't work out, they are a good option for everybody to find something to have fun with.

Minecraft, Raft.

Core Keeper and Terraria have especially low requirements.