Wave system in use, in the video below.
Source: Wave Systems
However, if the tracker falls off or is made to fall off, the parents will not be getting any kind of notification. The CEO at Wave Systems has claimed that research is still going on to develop a v2 tracker that would determine that it is active or has fallen off. Currently, a commercial version of this system is already being used in several parts of the US. This commercial device can monitor up to 100 trackers at one time. The price of the Hub is around $699 whereas each individual tracker is $99.
The Swimming Wearable Tracker by Wave System is unique and one-of-a-kind. Looks like a promising gadget with long-term applications in the swimming sector.