No one wants their young kids to fall into an unattended pool. It's (one of) a parent's (many) worst nightmare(s). iSwimband is designed to let you know immediately if the worst should happen. And probably also to keep your kids from making any friends.
The concept of iSwimband is simple: it's a water-detecting Bluetooth sensor that, when attached to your progeny, will send an alert to your phone if they're in the water. For the littlest ones, it can work as a bracelet and be configured to send out an alarm if there's any contact with water. For bigger kids, it can be set to only freak out if the contact with water is severe and prolonged enough to be troubling -- perhaps while playing around with a throng of other children in the deep end of a public pool.
The sensor and headbands are currently available for a cool $US100, a small price to pay for having peace of mind -- if, for some reason, you really do have to leave your offspring alone in the presence of a basin of water large enough to drown in.