Reading these recent comments from @StaceyPowers and @kingpotato, a thought just occurred to me. When the PS4 launched, one of its launch titles was a game called Knack, if anyone remembers it. This was and 3rd person action/adventure with a rather unique co-op mode. Player 1 would play as Knack whilst player 2 would play as Robo-Knack, a character to help out in combat with infinite spawns, didn't appear in cut-scenes and couldn't trigger story progression on it's own. It was also down to the real Knack to trigger both character's size changes.
This co-op mode was pretty much panned by gamers and most critics, with due respect to them, they did so because they failed to understand what it was for. A child would play as Knack whilst a parent, guardian, older sibling or whatever would play as Robo-Knack to help them out in combat, with puzzles and perhaps shoe the younger player get through platforming sections. Now, I did play through the game with my son who I believe was 5 at the time and had great time. Games like that are a brilliant idea that, sadly, haven't caught on.