Award-winning children's novelist Richard Peck cheats at first person. There's no other way to put it. Sometimes the first-person protagonist describes the thoughts or feelings of another character as if it were known fact. In lesser hands, these oversteps would be circled in red by the editor as careless errors of Point of View.
The more I think about Peck's cheating, the craftier it becomes. His first-person protagonist can't see inside everyone's mind, just a few people very close to her. To test this approach's viability, I consider my own life. Often I can tell by the face and body language of my husband or a parent what he or she is thinking, no matter what he or she actually says. So it's true: for people with whom the main character is very close, a limited first-person omniscience is arguably correct.