Hello!
It is up to you what you want to follow if you do so according to the legal rules. But if you put an AirTag in your child's backpack, your child is at school or on the bus and the AirTag is not linked to your child's iPhone then other people with iPhones could possibly get a notification that they are being tracked with the AirTag that is in your child's backpack. For example, it could be that the teacher gets a notification because the teacher is around your child all day.
I understand your concern for theft, but I don't know if this is the right solution. If you do anything in the way of tracking, always make sure you are doing it in accordance with the laws of the country you live in.
Possibly this article will help you on this topic of tracking:
What to do if you get an alert that an AirTag, Find My network accessory, or set of AirPods is with you - Apple Support