I was in a car wreck late JAN of 2011. As a result, I've got 2 small Metal brackets in my left forearm, and a steel rod the length of my left thigh, plus Metal screws in the brackets and rod both. I've got a card from my doctor, for in case I need to fly (so far only 1 round-trip flying since the wreck), so the people working the metal detector know What Exactly is setting off their equipment.
I usually leave my car/house keys in my hotel room, but I always carry a pocketknife. I carry/wear a backpack at GC, but I've no idea if that has any metal in it or not, aside from the zippers. I've got some metal in my belt buckle, not sure if it's enough to set off a detector or not though. I also wear a wristwatch and have a "Smart" phone. I've also got metal-framed eyeglasses, but again I've NO idea if those would set off a Detector or not.
But I'm only ONE person. I'm also not a Cosplayer, or someone bringing in equipment to run a game.
Figure 5 minutes to run me through the Detector. My Friend/Brother also has a backpack, phone, watch, belt, maybe his keys. Not sure if he has a knife. He'll probably only take a minute or two.
And there will be HOW many Attendees PER DAY to "run though" a Metal Detector? That will find . . . nothing. All to make 3-5 children "Feel Safe"?
If you're afraid of large gatherings, Gen Con may not be the place for you.