True - plus that way if you lose the kid, at least you'll have the backpack for their little brother/sisterWhy not both? That way there's redundancy. Plus, an AirTag in the backpack will mean that the kid can find his backpack if he misplaces it (if he gets an iPhone or something).
Seriously though - it's not a good idea to put an AirTag in the kid's pack, if the kid doesn't have an iPhone paired to it. It's going to start broadcasting to everyone around it with an iPhone or Android that "Hey, an AirTag is following you!" - even if you disable the chirp, it'll still irritate every teacher, bus driver, older kid with a phone in close proximity to the child.