I imagine it all depends on whose innocence they are talking about, and what they mean by the word anyway. It came to us from the ancient Romans, whose nocere meant "to harm", nocens was "harmful" and innocens its opposite; and it's certainly true that Charlie Brown never did any harm to anyone, despite his tendency to be put upon by everyone he meets. The Latin was more positive than its English equivalent - it signified integrity, moral uprightness and a Solomon-like uncorruptibility. For medieval writers innocent seems simply to have meant "without sin", which, though secularised, is what lawyers still mean by it.
However, there's another side to it, unrecognised by lawyers. The legal tag says ignorantia legis neminem excusat - ignorance of the law is no excuse, or to put it another way, you may be guileless but it doesn't mean you're guiltless. Yet absence of guile is the layman's understanding of what innocence is about. The infants who are supposed to have been murdered on the orders of King Herod were called the Holy Innocents because they fitted both sides of the word - they had committed no crime, and they were too young to know what was going on. The shades of the prison house, in Wordsworth's phrase, had not begun to close. The Genesis myth makes Adam and Eve innocent until they taste from the fruit of the tree of know- ledge. There are no grown-ups in the Peanuts strip, which may have been what tempted Professor Inge into talking about innocence.
But to think of Charlie Brown as "harmless" would not be quite accurate. It's a word sometimes given to half-wits, who were also called innocents once, or to those whose intelligence someone wants to insult. Old Charlie knew the score all right. He just didn't bear malice, but he'd surely hate to be called harmless.
For mere harmlessness, as opposed to all the righteous things implied by innocens, the Romans had innocuus and innoxius; my Latin isn't good enough to tell the difference. In English, I take it that innocuous was originally meant for non-poisonous snakes and suchlike, and used of humans only to disoblige them. Its opposite is obnoxious, a curious word, because for about 150 years it could mean two entirely different things. Ob- in Latin means "in the way of" and an obnoxious person was "in the way of, or path of, harm", in other words vulnerable, but then people began to think that obnoxious must be an intensified version of noxious (now hardly used except about fumes). There must have been some misunderstandings among 17th-century gentlemen.