For me, Kodi Smit-McPhee's performance seemed perfectly fitting with who his character was: a young, preadolescent boy who'd grown up in the wake of devastation. He inhabited the role, and any qualms with his performance – any hints that it was devoid of emotion – are probably more attributed to the simple fact that children have different personalities, and show emotion in a wide range of ways.

If you don't think kids act like that, then (as somebody who works with kids) all I can suggest is that you need to spend more time with a wider range of kids.