As technology improves, so do possible answers to questions like this.
New products are on the market that allow you to have Bluetooth tracking via a sticker.
The first "Bluetooth tracking sticker" that comes up on a Google search at this time is StickNFind.
- It's about the size of a watch battery (which is the kind of battery it uses).
- The battery lasts up to a year.
- It has about 100 ft line of sight range.
- You can use the product's app to track the device when in range, or use the app to get notified when the sticker is out of your desired range.
- It's currently $50 USD for a 2 pack.
- It only has 100 ft line of sight range, which might reduce it's effectiveness for older children whom are allowed to roam more.
- It requires a smartphone, or certain tablets.
- You need to have the app, and Bluetooth, running on your smartphone, reducing your device's battery life.
- Children lose small things, so you have to find a way to "secure" it to them.
Because the device is so small, it can easily be tucked into a child's pocket or hidden beneath their watch (or some other jewelry with a large face). If desired, it wouldn't be difficult to sew special patches into younger children's clothes to keep the item from falling out. Personally, I'd go for making a pendant they could comfortably wear around their neck and under their shirt, and design it to disguise the device.
I'm a heavy reader of tech-related news, and when I heard about products like this my first thought was to tracking children, not objects. Normally, our son is never far enough away from one of us that we fear for his safety, let alone fear him being abducted. However, if we were to ever take him on a family trip to somewhere crowded (especially if we we going on the trip with others), then something like this could definitely provide peace of mind. It takes half a second for a young one to run off. When you're in a big crowd, and you have other friends and family with you, you're going to get distracted for half a second at some point. While most times your child will be perfectly fine, there's a wide variety of things that can happen between losing and finding a child, not just abduction.
I would add, though, that this suggestion, like the others, is more about easing the parents' worries than increasing the safety of the child.