Which Lego games are best for couch co-op?

I am not 100% sure but aren't they all limited to 2-player coop?

Other than that I'd say it probably depends more on what franchise they might enjoy best, since the gameplay itself is quite similar between all of them as far as I know.

Are the Star Wars, Harry Potter, Hobbit, Lord of the Rings and Indiana Jones ones not an option? I think there's also a Pirates of the Caribbean and a Jurassic Park one.

I personally liked Avengers and Super Heroes most of these games and Ninjago felt a bit frantic, not sure why... I've had quite a few kids play Batman 2.
I'm not sure if it's any of there but I remember a friend of mine telling me that he preferred the split screen in the older LEGO games as they did something more clever about what happens if you are not on the same screen together, but I couldn't say if that was relevant to playing or just his sense of artistic style.
I would probably think about what they enjoy, thematically and play style wise, the newer ones are generally more open world, while the older ones are more of a "pick a level" style. Oh, and as WaxWorm said, they are all 2 players max. Except for Ninjago, which I think has a 4 player battle mode.

The last thing that comes to mind is the question if you expect them to really play the game properly or just mess around to kill some tie, because the second one works better if you already unlocked some stuff for them beforehand, else they are very locked in.

You can't go wrong with Marvel Super Heroes.
Undercover's humor might go a little over the head of kids as a lot is based on cop movie parody.
Batman is just not as fun in co-op as the Marvel or Star Wars (or LOTR!) games
Ninjago can be a little harder for younger kids and Avengers is kinda boring.

LOTR and the first Star Wars games are the best but kids might not be as into it as they would be into Marvel I guess

Yeah any of those games are good. My son loved Undercover, so I'd say it's between that and Marvel heroes as the first pick.
Other games you have that my kids enjoy playing with me:
Avicii Invector - Random I know, but it has a beginner mode and the songs are fun
Bee Simulator - 2 Player co-op only, but not too tricky for kids to play
Contraption Maker - Single player only, but one person controls and the others give ideas and it works alright
Fling to the finish - Requires some dexterity, but my 4 year old and I team up and manage to get to the end
Go home Dinosaurs - Single player, but also a big hit when my kids were younger if you help them along
Lego Worlds - 2 player co-op, no real goals, just build and explore
Moving out - Requires some dexterity, but you have options to make it easier. I play it with my 4 year old
My Brother rabbit - Single player, but can be a co-op experience if you have a few kids gather round the screen looking for hidden objects
Portal knights - For the older ones maybe, build and explore with easy combat
Ticket to Ride: First Journey
Tools up - Similar to moving out
Tricky towers - Some skill required and it is competitive, but can be fun

You also have some of the Humongous Entertainment games like Fatty bear, freddi fish and putt-putt. Single player, but still a hit with younger kids. There's a few others are well that might be fun in a single player / shared setting