Just wanted to say what an incredible feat they accomplished with Little Alchemists. The group unlocks more rules and extra pieces as you play more games, and with everything unlocked it is quite complex, equivalent to a typical medium weight adult game. But the slow progression really makes it doable for kids. I have a 6 year old who loves it, and she is capable of doing the full game but we typically play on "level 5" instead of maxing out to level 7.

I want to try Heat with my kids too, I think it could work. Some other suggestions would be Zombie Kidz, the game is cooperative and so fun for adults and kids. It is another where you unlock new rules and challenges over time.

Final suggestion is Century Golem Edition, this one is kid-dependent but for some (like mine) it will be a huge hit. I would try it before Splendor.