Yeah, Mario Galaxy games are solid for 2nd player as a helper.
I think the GC mario kart game had a coop mode. Mario Kart Double dash. Player 1 drove and player two would toss out weapons.
New super mario bros is a good coop with kids as long as they are cool with repeatedly dying. Any player can just play and die, and they end up in a bubble, allowing you to keep going or pop their bubble to get them playing again.
Lego games are good with kids 4+, you sometimes need to swap characters or reign them in to simultaneously pull a lever with you, but they can pretty much do their thing and it won't slow you down.
If you want to go way back, Bruce Lee was a great helper game. P2 can be the fat green sumo guy and help you beat up the ninja. THere are a number of screens that didn't include the sumo or ninja but that never bothered my kids. They just liked having a character they could control, and it never impeded the progress of P1.
And honestly, my kids love watching me play RPGs. They participate by helping me pick my party, make decisions, spot treasure chests. SUrprising how much they get into it as a group activity.