The LoJack system is also definitely effective. Since its inception, more than 100,000 vehicles have been recovered worldwide -- with over 60,000 recovered in the U.S. alone. This amounts to roughly $2 billion or more in recovered stolen assets to date.
You are right about the limitations in RFID, since identifying signals emitted by these devices only go about 30 feet or so because of battery and power limitations. If you were to use a bigger battery then the tag would have to be bigger and couldn't necessarily be produced by specialized RFID printers because of the bulk. Also, the majority of RFID tags are passive in nature, which means they only come to life and start broadcasting when activated by a compatible reader. This means that a missing child would have to be in range of a reader in order for a device on his or her person to be activated. (In LoJack's case, the devices are activated by the police once you report the car a stolen).
The LoJack creators actually did have a plan to develop a child-safety version of the automobile antitheft device, which might be hidden in a bet buckle or shoe heel. However, privacy advocates were appalled at this suggestion way back then, so the idea was dropped. Now, however, the tides have shifted as we live in an era of multicolored public safety alerts and Amber lost child networks. So, there are a few devices on the market that can be used to broadcast the whereabouts of a missing child, or signal loudly if a child wanders too far away from a parent. In this new security-conscious world, electronic leashes are suddenly okay.
In fact, cell phones and advances in wireless networking may eventually serve that purpose, since mobile phones essentially broadcast where you are within a city block, and there are voice-over IP devices that employ location-aware technology and can be worn much like a piece of costume jewelry.