We don't have any background on this case, so any conclusion is premature. What if his "pals" forced him into the sand, and then when the kid stopped breathing, they covered their asses by blaming Naruto and claiming the kid "wanted" to be buried alive. Head first.
Also, what do we know about the kid? Was he prone to imitating cartoons? Has he ever tried to put the cat in the washing machine after watching Tom and Jerry? I can't believe (and you can't honestly claim to believe) that going from "never trying to imitate" to "wanting to be buried head-first into sand" is a small and logical step. And why, why haven't the parents told him anything when they noticed the trend? It takes a whole of five minutes to tell the kid that what they see on TV isn't real. So his extremely irresponsible and negligent parents didn't even notice his behaviour; either that, or they didn't care. That is, if he really was prone to such stuff. And if he wasn't, well, see theory number one.