An accelerometer would be able to detect the unique thrashing signature of a body trying to stay above water; it differs greatly from swimming or kicking to stay afloat.
this is great, but it wont save any lives if no one is around or if the kids are expected to be playing in water. There was a death at a day camp in my neighbourhood last year; the kids were under watch, adults were swimming with the kids,but because of all the activity (outing at the pool for day-camps can get really crowded) the life guards did not spot the kid in distress and he drowned. A device like this that inflates when distress is detected would have save that life.
If they want to actually sell any they should put a video demonstrating the product on the front page and not limited by being Apple Quicktime only nor hidden behind a "super nanny" logo.
I think it just activates an alarm when it comes into contact with water. It's not a floatation device as far as I can see.