My younger cousin comes over today (like every few months) and we will be playing some couch coop games over the weekend.
So far we tried a lot of games and found some great ones along the way. Sidescrollers work sometimes, but third person and isometric is better. And I won't touch another twin stick shooter haha.
Do y'all have any suggestions, it's always so much work to find a good new game (as opposed to just going after steam review, buying, and returning within 30min of playtime because it sucked).
Honorable mentions:
Swords of Ditto (literally no one plays this, but it's a pretty neat co-op zelda-like
Darksiders Genesis (hack n slash)
It takes Two (co-op puzzle)
Children of Morta (great roguelite)
I also bought Secret of Mana, maybe we'll play this.
Super Mario Galaxy 1/2 - it sounds like your cousin is probably too old for this but this has one of the best "younger sibling" modes. They help collect and shoot bits. Which ultimately doesn't matter much
The Legend of Zelda 4 Swords Adventure - It's basically multi-player Link to the Past! Bonus points if you dig up a GameCube, link cables, and GBAs
Kirby Super Star - a mix of competitive and coop games
Super Mario World (any) - you take turns but if you don't go 6 levels without dying to his one it can be fun
Halo 1-3 + ODST - Great classic shooters, the campaigns are customizable for difficulty, and a little kid probably loves alien shooters
Borderlands 1/2 - Looter shooters with a sense of irreverance
Dead Rising 2/Off the Record and 3 - Zombie hoard murdering with combo weapons
The Simpsons '91, Battletoads, TMNT, Streets of Rage, etc - Beat Em Ups are great for multi-player