Well, since the others so far are playing along with what you want to hear, I'll say, if you think it was no big deal, then sign the dang form and act like it. The form admits to the 'sin' of not telling an adult. That's true, he didn't. It isn't a false statement.
Making a huge deal out of this will serve no purpose. None.
I differ--it serves the purpose of letting the child see you will defend him--even if he is wrong. The others are right--he did not tell on the boy with the knife and 'show responsibility'. My bad! CYA is today's game and has also created a dog eat dog world. Too bad it just didn't end where it began because it seemed innocent enough to me. I do question a 9 year old with a knife anywhere except at a Boy Scout meeting.