Knowing how to keep kids safe is never easy. Ideally, you'd want to provide as much realistic safety as possible without being disruptive or invasive, but that can be a hard compromise. This backpack may not be a compromise at all: it comes with princesses or Avengers on it and will also stop rounds from a .44 Magnum.
The back panel of Amendment II's kids backpacks is lined with its "RynoHide" armor, which is "light as air, stronger than diamonds and can be as flexible as rubber" thanks to a proprietary mix of carbon nanotubes. It's thin and soft, meaning that your kids will never notice that it's there, but it's capable of stopping bullets from most handguns.
Amendment II suggests that "the backpack can be quickly brought to the front as a shield or can serve as center of mass protection while fleeing the scene of the shooting." It also appears that the armor itself is quickly removable through the bottom of the backpack, where it can be unfolded into a much larger blanket to better protect against something like a bomb.
Amendment II's ballistic backpack is available for $300 with armor that will stop .45 caliber or 9mm rounds, with an optional $100 armor upgrade to stop .44 Magnum rounds.