That's something completely different. In that case he was stealing his parents' stuff, not something that was technically his. Although I really could see your point, if, say, he stole one of the presents intended for his mom or something.
That's why it's the parents' responsibility to keep their kids from opening presents early if they don't want them to. Getting the police involved was probably the dumbest thing a parent could do. Now she's probably gotten the kid a criminal record, and for what? All because she couldn't discipline her own child.