Minecraft, if you play online and enter a server like Cubecraft the default lobby is very kid friendly and has a large interesting map to explore. And of course the game itself has tons of single player content and you can build your own stuff in survival or creative modes.
Try Grand Theft Auto 5 and see what you think. If you don’t actually do any missions it’s really a very nice and alive world to explore. The pedestrians make some rude comments though.
Also Arma 3, the base map is a sandbox of a real life island with most buildings enterable, and no people or traffic or anything offensive by default. You can enter the editor, place a civilian player and some vehicles, and explore the island completely peacefully. There are also lots of mod maps some are very good.
If only driving around is acceptable then the Forza Horiizon games are great for driving around.