Where the backpack goes, the kid goes. And the associated battery, case, etc would be easier to put on a backpack than on the kid's clothing.
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.