Minecraft / Terraria, 100%. Both games have available co-op, so they can play with their friends, and allow quite a bit of creative expression. Terraria is made by some of the best indie devs ever, if it helps, having been getting free updates for years, even though the devs said they had finished the game years ago.
For a more socially open experience, I could also recommend Sea of Thieves. It’s a game about manning a pirateship and collecting loot, where you can sometimes run into real people on their own adventures (or get your ship sunk by them, after all, they’re also pirates). There is co-op, too. A cash shop is available, but all items are strictly cosmetic, with many items available through gameplay alone.