Solo Leveling is the ultimate 'junk food' read, but it’s the high-end, gourmet kind of junk food. It doesn't pretend to be deep literature, but it executes the 'leveling up' fantasy better than almost anything else out there.
It’s a great 'gateway drug' to get kids off screens and into physical books, even if those books are full of shadow monsters and glowing blue leveling screens. Just be aware that it’s violent—if they aren't ready for a bit of gore, maybe stick to something like 'Spy x Family' instead.