Bloons TD 6 is an absolutely wonderful family-friendly Tower Defense game, and if you don't own it already you can grab it free on Epic Games and it works great on Linux.
Easy enough to download it and run it thanks to the Heroic Games Launcher too. And to test, since I already have it on Steam, their cloud login system perfectly synced my progress across so that was nice to see. Running it with Wine-GE-Proton8-13, which you can download directly inside Heroic has given a perfect experience on my desktop.
Bloons TD 6 is absolutely madness once it gets going. It's a weirdly theraputic game, watching various different types of balloons getting popped by monkeys. What a wonderful idea for a game. So simple but it's hard to tear yourself away from it once you get going, you just want to play one more map.
There's quite a lot of content you get with the game too, and it can be a thorough challenge on some of the later maps. Don't be fooled by the easy start on this one!
Go grab it free on Epic Games. Seriously do it, such a great game. Have enjoyed a lot of hours of this one, and lots of that in co-op with friends and family.
You can also grab Loop Hero free too, another great game!