The best multiplayer game: Pixel Gun .

This game is a very good online game for any child and parent of any age.
The game comes with the ability to play a campaign, make servers (with and without the password), create a fun space where you can meet and chat, create castles and clans.
The game allows duels between teams, conquering the flag, choosing a pet, customizing the player with weapons and accessories, creating a space for each player. You can also create friends, invite and like them.
Pixel Gun 3D has two modes: a single-player Survival Mode and Multiplayer Mode.
First the single-player can be start with with a multi-level campaign and an interesting comic book story.
Into this story, you run around shooting mostly undead enemies like zombies and skeletons and earn points.
The Multiplayer Mode where you join others in a shoot-or-be-shot contest come with many servers and options to do that.
You can also create your own skin, update your clan castle and environment ( like pet house).
I could enumerate the options and features of this game but let you discover it.
I've spent hours playing this game and it's one of the fastest and does not require a lot of resources.
The game can be found here.