Kids ages 8 and way, way up spend 21 million hours a month in the Roblox universe. It’s a massively multiplayer online game where friends can meet, collaborate, chat and compete. Everything has a Lego-like look, so even fierce battles are cute. And the best part is that players can construct their own games and worlds using a surprisingly sophisticated tool kit. It’s O.K. to call it a junior-size counterpart to Minecraft — as long as you know that Roblox came first.