Horrible tragic accident and my heart goes out to their families.
Water should always be treated with respect, and those that play on it be both equipped with the right gear and also prepared for accidents happening. I paddle a canadian canoe, and although it is an amazing hobby both for ourselves and the kids, you have to make sure you are within your ability and take extreme care in even the calmest conditions.
Some modern open canadian canoes do have natural buoyancy (double hulled, royalex/plastic) but fibreglass ones dont and even with natural buoyancy if you are swept into cold water suddenly, it can be very difficult to get back into the boat. It also sounds like the canoe was overloaded. Unless the water and wind were flat calm and unless my boat was airbagged (adds buoyancy) up to the hilt, and i ws wetsuited and lifejacketed (not BA'd) I wouldn't take it on the sea. I definately wouldn't take kids.
Lifejackets should keep your face out of the water if you are knocked unconsious. BA's just help you keep afloat. Kids, even paddling in canals, should always wear Lifejackets.
As one of the fastest growing sports in the uk, and with more and more families taking to the water, there should be more education about water safety in and out of boats and what to do if things go wrong.
Poor poor kids, and poor poor families, :(