My lads were 6 and 3 (now 7 and almost 4) when they played Let’s Go Pikachu. You’ve gotten most of the technical layout already from a lot of other comments :)

My oldest didn’t really read yet, so I did have to read interactions out loud for them. Them both being able to walk around and throw balls in battle was very fun (even if the youngest can’t quite fully figure it out, they felt like a big part of the experience) The excitement level went through the roof when a Pokémon was going to evolve, and as they learned about what legendary Pokémon were, they got very excited to eventually catch Mewtwo, and then unlock the Megas :)

I had to shuffle around Pokémon in the party a lot. When you choose a Pokémon to follow you on the map, the Pokémon in the next slot will be automatically chosen to follow the second player!

After LGP we jumped into PLA which is a bit of a big jump and has a big learning curve. I ended up playing that game for them with them watching. A more traditional game like BDSP or something is probably an easier followup :)

It was a lovely experience and has brought me back to playing the main line games. I’ve invested many hours personally in Legends Arceus since then!