If this is true (and it might be), it would be only because most adults are completely oblivious when it comes to children. Adults in our society are notoriously good at seeing only what they want to see (I'm not talking about the HN type of person, I'm talking about your general population adult), and often this means overlooking certain characteristics in children. Or only seeing aspects of ADHD, or bipolar. Or not seeing that a parent is a really awful parent, etc.

Kids generally do not have as many biases as adults, and as a result they have less expectations. They don't see their peer as some angel, because they don't expect him to be an angel. They don't expect anything at all, so their perceptions aren't clouded.

But in general adults are much better at picking up on things like social cues and odd behavior.