Here are some examples from other areas to illustrate what is wrong with your use of statistics.
Let's say that a study showed that in an average year, 95% of the people who die in Germany are German citizens. Does that mean it is much more dangerous to be a German citizen in Germany than it is to be a foreigner living in Germany? I hope it is obvious that probably 95% of the people living in Germany are citizens, so it would not be surprising that 95% of the deaths would be to citizens.
Let's move a little farther from the obvious. Some people on ScubaBoard have pointed out that in the last 10 years or so, roughly 50% of the people who have died while cave diving were certified cave divers, and roughly 50% were not. They argue that this shows that there is no benefit to cave diver training, since certified and non-certified divers divers are dying in equal numbers. That argument assumes that an equal number of certified and uncertified divers are going into caves, which is decidedly untrue. With all the restrictions in place and all the warnings in place, I suspect that more than 95% of the people entering caves are certified cave divers, which means that the percentage of uncertified dives who die in caves is huge in comparison to the percentage of certified divers who die in caves.
Similarly, your argument about the number of pool-related deaths during professional instruction compared to the number of deaths with children being taught by family members in pools assumes that the number of children being taught by family members in pools is comparable to the number of people being taught by professionals. That is absurd. As an agency, PADI alone certifies nearly a million divers world-wide a year (all levels). Now, what percentage of families around the world have pools in their back yards? What percentage of the families with pools in their back yards have members who are divers? What percentage of that subgroup would be willing to teach their children in that pool? When you consider all that, it is likely that only a small handful of children are being taught by family members while in their personal pools.
You have made a second assumption, and that is that a random family member is more likely to provide a safe training environment than is an instructor who is trained to do that. Look around you the next time you are at some sort of dive resort and observe the average skills of the average divers. To what percentage of those divers would you entrust the lives of your children?
As I have said repeatedly and will say once more, the risk for this is low, but it is not zero.