Why is it desirable to track the kids backpacks? Are they so very untidy that they often get lost?
Of course, this unreasonable fear of abductions means that a commercial GPS tracker for kids would sell much better if it supported the "abducted kid" use case well, on top of more likely use cases.
Unless you talk about the wilderness.
Actually when I let my kids go outside alone, I usually give them a phone. I live in a big city. Reception could be a problem in the countryside.
They would typically be worn by the child, and thus a good indicator of where your snowflake is.
But I want to use this to put on BIKES!
Are you thinking about theft tracking or some other use case?
Though I have to say I saw a youtube video recently (sorry to lazy to google it) where someone put an apple tracking device in a bike and was able to locate it after it was stolen.