But what scenario do you want to prevent? I think in case of abduction, the backpack would likely be left behind.
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.