It’s a FOSS voxel engine, so they can play multiplayer with their friends for free. MineClone2 is a Minecraft clone on Minetest, so even if their friends don’t have Minecraft, they can still play with friends.
For paid games, consider indie games, as they’re less likely to be micro-transaction bullshit. Raft, Stardew Valley, Two Point Hospital are good options. Not so much multiplayer, but Stardew supports it. Multiplayer wise, maybe Among Us if the parents are comfortable with that.