AND the backpack would only be good it the shooter was behind you. The kid would still have three sides unprotected. Not sure that's a good investment.
If I remember the kids at the Florida High school didn't want see thru backpacks that were going to be required after the shooting. They just wanted to take out guns.
The kids would have to be trained if shooter in front to take off backpack real quick and put in front of their face/chest to use as a shield.... I doubt many 5-6 year olds would know better?
