Cold water lengthens the survival time by two mechanisms. It triggers the mammalian diving reflex, which halts breathing and conserves oxygen by slowing the heart rate and moving blood to vital parts of the body. This response is stronger in children than adults.(14) An opposing “cold shock response” may predominate, which leads to a faster heart rate with potential fatal rhythm disturbances(15). This response also causes immediate aspiration and swallowing of water, which quickly cools the heart and carotid arteries leading to “selective brain cooling”.(4) A reduction of brain temperature by 10° C decreases energy consumption by 50% and doubles the duration of time the brain can survive without oxygen.(16) This “therapeutic” hypothermia is accelerated by surface cooling in children and small adults with higher surface-area to body mass ratios and less subcutaneous fat. Panic by the victim (breath holding and vigorous attempts at escape) and protective gear worn in cold water work against these principles and may prevent therapeutic hypothermia.

http://www.sitezed.com/an-analytical-look-at-survivable-subm...

Apparently if a child is very young, the water is extremely cold, and everyone involved is extremely lucky then crazy stuff can happen. (Un?)Fortunately this isn't something that happens often enough that we can get a lot of data on how the body can recover from stuff like this.