Not a co-op game but you could try Shattered Pixel Dungeon. A rogue-like in the classic vein but with graphics and rules that make it approachable. My kids could handle it at 10 years old, but they did consider it "hard" then, and even still do a few years later. Open source, free on F-Droid (but donate if you like it), $5 on Apple app store. Actively developed for years. [Edit: Should mention that this is my favorite casual game on my phone, for many years.]

It does support seeds, so you could make it more of a mutual experience by both playing the same seed and talking as you're playing. That's not co-op, but not completely solo, depending on the kid.

There's never been a better time to vibe code a game to play with your family. If you use Claude or Codex to put the game together, you can make your own entertaining casual game that works great remote and has no dark patterns, data harvesting, fees, etc.

I've spun up a personal chat server to escape the horrors of Discord for my family and friends; I've spun up a tile-based RPG game that I have been independently developing not as a commercial interest but simply as a digital space belonging entirely to me.

Creating your own non-enshittified software oasis is the killer app for AI right now imo.

Most games are cluttered with micro transactions and unethical data harvesting. You should look for indie games instead of the big scam companies, stuff like Voodoo was always there, low effort apps cluttered with ads and paywalls. Some of my favorites are: Terraria of course - it's a timeless classic game with so many options, looks boring at first but you have endless options. Second is The Past within, it's a clever co-op puzzle game built entirely around communication. You are solving the same mystery from two different time periods, sharing clues to unlock each other's puzzles. Zero ads, zero microtransactions. I really LOVE Bloons TD 6, it's a tower defense game where you both place monkey towers on the same map to stop incoming balloon waves. BTD6 has IAPs, but they are purely optional skips. You can earn every tower, upgrade, and map just by playing normally. There is still Minecraft of course, but its Microsoft which is meh... The last one on my list would be Human Fall Flat which Ive never actually played myself but it was sitting on my wishlist forever, never had anyone to play it with tho, worth checking out. If you want to bypass app store garbage entirely, another route is looking at Apple Arcade or Google Play Pass, which explicitly ban ads and microtransactions across their entire catalogs.