Minetest with mods & SuperTuxKart
Minetest is an excellent candidate for this use case, I'd say. I play Minetest on a Raspberry Pi 4 connected to a Minetest server hosted on a different Raspberry Pi 4 (both running Arch Linux Arm) over Wifi regularly and don't find latency or lag or anything to be a significant issue.
The world I've been playing in that particular way is on the game VoxelLibre (Formerly Mineclone 2). It's intended to mirror Minecraft's functionality as closely as possible. It's not 100% implemented, but a lot of it is implemented, and it's very playable. The default Minetest Game is great too. It lacks mobs, but mobs can be added with mods like the various Mobs Redo API plugins. (And it's easy to have mobs but not enemy mobs that can kill the players if that's the vibe you're going for.)
These aren't browser games, but could still be fun.
I used to play Battle for Wesnoth a lot as a kid, both with my dad and by myself, and as it's turn-based it should run fine on a raspberry pi.
OpenTTD could be fun, although it may be a little complicated depending on how old they are. It's not very difficult in terms of challenge though, it's more of a sandbox once you get past a certain point. There's also sandbox options to give yourself lots of money if you didn't want to worry about money and just build cool trains and stuff.
I used to play a lot of Supreme Commander with my dad as well, so maybe something similar like Beyond All Reason or ZeroK could work if they take an interest to that kind of game. I haven't tried them though.
Mindustry
Maybe https://spektor.itch.io/tachyon, but it's hard to recommend something suitable without an age range.
does an emulator count? like MAME or NES etc?
@Num10ck@lemmy.world that is fine, but I suspect it is not possible to play it in a multiplayer setting via a browser
Definitely possible -- check out emulatorjs. There are a few premade docker containers for it, but they were a bit finicky when I tried them. I eventually just ended up writing a react wrapper for it and have it hosted in a docker container. It works great, and will even play psx games without issue.
Romm might fit, if you've got the patience to map controllers. Remember, it's a BYO rom situation, but that's nothing the guys over at @piracy@lemmy.dbzer0.com can't help you with.
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