If I knew you (or your kids) personally, I might guess differently. But strictly from an outsider's point of view, I'm going to guess it was either your son or daughter, as well. The other scenarios just don't make sense.
It does seem somewhat strange that the TSA didn't think much of it when your daughter said she had never seen the knife before.
"Oh, okay, your bag must have just been tampered with by a terrorist then. Thank you, move along. That doesn't make a whole lot of sense, does it? (Then again, we're talking about the TSA here.)
However, I would be
very surprised if the TSA (or any other government agency) conducts tests by placing objects in unknowing subjects' bags. There are just way too many variables and risks associated with doing so, not to mention the practicality of actually getting the item there to begin with.
Thus, most likely scenario is that someone with access to the bag (your family) place the object there. Have you talked with the wife?
