Any other recommendations for board games and young children? I'd prefer a thematic game - nothing too abstract. One of my boys loves superheroes but they are too young for anything complicated.
I think you're right to worry about Mice&Mystics being too much too quickly for them.
I had success with my five year old boy (now six) with King of Tokyo - largely ignored the cards at first & modified the rules that three cubes could be exchanged for a point of health, but he thoroughly enjoyed it (and still does). Lots of theme, lots of great artwork. I think this is exactly what you're looking for to be honest.
Monza (by Haba) is a kids' racing game that plays a little like Formula De - roll dice to figure out movement around a track. Another huge hit.
Pitch Car is way expensive, but it's absolutely solidly built & a massive hit with every one of his friends that we've introduced it to.
Carcassonne was a great way to introduce "following the rules" for tile laying, taking turns etc. We rarely play it for points, even now, instead using it as a co-operative "let's build huge fucking castles & roads".
Frank's Zoo - card game featuring progressive ladders of sets. Introduces pecking order & trumps. Big family hit.
"Survive! Escape from Atlantis". No reading at all, lots of cute wooden sharks/whales to eat people with. Big family hit. Dripping with theme.