Local CO-OP/Splitscreen games to play with my little brother on PC?

I have a little brother, he's 6 but didn't play games that much, I would like to hear your suggestions what games should I play with him? We have 1 PC (so no multiple PC), Keyboard + Mouse and 4 controllers. Preferably a simple game. And one that doesn't require difficult manouvering, for example, Overcooked is difficult for him. But any suggestion is welcome.

So far we've played:

RACE:
Dirt 3 Complete Edition
Dirt Showdown
GRID 2
Split-Second
Pressure

FIGHTER HORDE:
Dynasty Warriors 8
Samurai Warriors 5

FIGHTER:
Mortal Kombat X/11

SHOOTER:
Gears 5
Resident Evil 6
Serious Sam 2/3

TOP-DOWN:
Lara Croft & The Temple of Osiris
Tesla vs Lovecraft
Alien Swarm: Reactive Drop / Alien Breed Impact
How to Survive (way too complicated)

ADVENTURE:
LEGO Star Wars /Lord of the Rings/ Pirates of the Carribeans (Best was Star Wars complete edition & PotC, the newer ones are too complicated)
The First Templar
Overlord 2

PUZZLE:
Portal 2

BOARD GAMES: (always better in person)
Risk: Global Domination
Catan
Fury of Dracula

ARENA:
Overcooked! All You Can Eat!
Gang Beasts
Pummel Party
Move or Die (way too difficult)
Rock of Ages (1, the rest are getting too complicated)

PLATFORMER:
Guacamelee!
Never Alone (Kisima Ingitchuna) (Beautiful game)
Trine 1&2 (Beautiful and simple games, with a small amount of fighting, this one was perfect)
Unravel Two (helpful coop with puzzles, another great game)

Something simple & fun, that doesn't require much thinking. So far we had the most fun with platformers, they are simple enough, but also with race games. Extra amazing if it's +4 players since my girlfriend and/or parents could play.

Thank you!