Many people have had the fear of losing their child in a busy place or perhaps having their child kidnapped. What type of technology can I use to locate my child if they get lost or kidnapped? I'm not necessarily looking for a specific product, but rather a summary and comparison of the types of tracking products that can be used.
The factors that I would consider important are safety, cost, practicality for children ranging from infant age until 6-7 years old, and ease of use for the parent.
Update: I took out 'GPS' from the title to make the question more generic.
Update 2: Another factor that is important is concealability. How easy can the device be hidden on the child without others knowing about it (perhaps including the child themselves).