Your situation sounds perfect for Dungeon World
Dungeon World is a world of fantastic adventure. A world of magic, gods and demons, of good and evil, law and chaos. Brave heroes venture into the most dangerous corners of the land in search of gold and glory. - Dungeon World p. 7.
Dungeon World relies primarily on d6s
You need a handful of dice other than six-sided, but you can use an app or online dice roller for that - the vast majority of rolls in DW are six sided dice. All checks within the system, including making an attack, using a class ability, and making general moves are resolved by rolling 2d6s and adding a simple modifier from your class sheet. The only time your roll a die other than 2d6s is when you deal damage (or a monster deals damage to you).
Dungeon World is driven by the story and the choices players make
It's a rules-light, quick to learn system where a player who doesn't know the rules yet (or has trouble remembering them, like a very young player) can still participate. It's everyone's job to listen for when any player describes something that sounds like one of the game's "moves", so he can just say what his character does and you guys can point out when it's time to roll dice - that's actually the game's core mechanic, not a "fix" so it works for kids. Instead of saying "I make a weapon attack against the orc!" a rules-laden phrase, players instead narrate the actions of their character: "I take a stab at the orc, swinging with all my strength" then the GM would declare that the player was making a Hack-n-Slash move and dice would be rolled or the Gm might decide there is a more appropriate move from the player's class sheet (players can also chime in to suggest a class specific move).
The entirety of the rules a player needs to play the game are contained on a few sheets of paper. There is a basic and advanced moves sheet that everyone uses and each class will have 1 double sided page (and an extra page if they cast spells).
Dungeon World's Creative Commons license means the core game is freely available on the internet.
The basic rules are available for free online, here (text only).
And you can download the Basic Moves, Advanced Moves, & class sheets for free from Dungeon World's website.