As for "lie-cheat-steal machines", I've personally observed it across thousands of children. It mainly happens at 11-13 years of age, middle school in the USA. When I worked for Southland Corporation (7-11), we tried everything to keep a store adjacent to a middle school operating. Finally sold it. Kids go through earlier phases of this when they first discover lying. Each kid differently, of course.
"On the contrary, kids are relatively complex emotionally and are capable of empathy from a young age."
That's not "On the contrary". Kids are, indeed. Emotionally complex and empathetic doesn't preclude naive innocence at all.