Suggestions of coop games to play with kids 10 & 12?

The Crew - Mission Deep Sea - card game with a simple trick taking mechanic. Difficulty is very modular as you decide a difficulty level before each game. Difficulty is decided by the numbers of missions taken and the relative complexity of those missions (this is all explained on the mission cards). Missions are based on which tricks you win, with simple rules like "I win no 1's" or "I win at least 3 9's".

Hanabi - Card playing game where you don't know your own hand. You describe aspects of each others hands (colours of cards, numbers on cards). Your goal is to place a pile of the cards 1,2,3,4,5 in each of 5 colours. Don't play with mathematicians.

• Marvel United: simple card driven Pandemic-ish firefighting
• Quirky Circuits: Penny & Gizmo's Snow Day!: programming, concurrent planning
• The Game: hand management
• Tranquility: hand management, base game is pretty simple, comes with a bunch of expansions to ramp it up
• Dice Conquest: Dice placement, manipulation
• Teeter Tower: dexterity, balance, stacking
• The Mind: more of a silent social experience than a game. A lot of fun when you "get it"
• Kites: Sand timers. I haven't actually played this one yet but looks fun

A bit more complex:

• Pandemic: The Cure: My favorite version of Pandemic, turns the game into a dice based romp
• The LOOP: This is somewhere between Pandemic and Spirit Island, fun theme, might be a good preparation for more complex stuff
• Regicide: Only needs a standard deck of cards, not easy but very satisfying when you get a win
• FUSE: realtime, can get pretty hectic. Check theme for appropriateness.

And finally a BGG search, filtered for 10+, mechanic:co-op and sorted by rank. Tweak it using the advanced search link on boardgamegeek.

5-minute Marvel is unsurprising a similar theme, but it’s frantic and chaotic.

By the name of the game, it’s “5-minutes or game over”. I switch away from a countdown timer to using a stopwatch when I teach it and simply have a leaderboard of “how fast can you beat the game”. That way the game doesn’t get disrupted in the middle and it doesn’t kill the momentum when the timer goes off.

any other games they do like? what works for one 12 year old won't for another.

Menara - cooperative dexterity game, where you building towers Legendary - Marvel Superhero deckbuilding . "Semi" coop, but if you tweak the difficulty, the focus is on coop Flashpoint:Fire Rescue - Firemen. rescue people, fight fires.
Pandemic/Forbidden Island - this is the 'classic' coop. it's a little puzzly for my tastes, but it's popular.
D&D Adventure Games - Wrath of Ashardalon, Castle Ravenloft. alittle older, but scratches a coop dungeon crawler itch.

my son (11) really enjoys Aeon's End and Spirit Island.

Maybe a little bit too easy for your 12 yo, but still worth a try:

• Stuffed Fables is a coop adventure game for kids where you have to make decisions together and talk about your feelings

• Zombie Teenz Evolution is an easy and fast legacy coop game where more rules and mechanics are slowly added over time

Stars of Akarios, if they're both into space games...

if you have multiple systems, did you consider minecraft?