If there is one game that any kid from elementary school age through high school is aware about, it would be Roblox. And this is for a few reasons.
1. Roblox isn't just one game, it is hundreds of thousands of games all rolled into one service. There is a game for any interest in Roblox.
2. Roblox is free... to a point. It is free to install Roblox and there are many free options in terms of games that a player can play on them. There is, however a way to pay for "Robux" which gives the player a currency that they can use in-game to buy new items for their character as well as a "Builders Club" membership that players can buy with real money to also unlock more digital items.
3. Basically any computer that has been released in the past decade can run it. The graphics might not be cutting edge, but that is part of the reason why it is so popular, it doesn't put insane demands on the systems that the games are played on.
Roblox is mainly a resource that allows people to make their own games. When you make a game on Roblox, you can choose to monetize it if you see fit. This could be allowing players to buy items in your game or unlock new content. Any purchases are split between the company and the game developer in that scenario.
This being said, there are many games on the platform that do not have real world purchases and can be played entirely for free.
Here is the ever-growing list of games for Roblox. Roblox can be played on a multitude of gaming systems. Microsoft Windows, macOS, iOS, Android and Xbox One all have Roblox support and allow you to play Roblox games on them. And, there is cross-platform support so players can play games with their friends regardless of what console they are playing on. Xbox players can play with their friends who are playing on their iPhone and any other number of combination of consoles as well.
Roblox is aware that their userbase reflects a wide range of ages and there are a plethora of parental controls that parents can take advantage of so that their child can enjoy the gaming service properly. Here is the Roblox link to their suite of parental controls and resources.