Nick, I wish children were all that rational. I work in an ER, and I
have seen the sad proof that sometimes, children don't get out of
situations that perhaps they could, for one reason or another. I've
seen poisonings, drownings, crushes, falls, etc. Children can be hurt
very badly by some very simple circumstances. They just don't think,
and they don't have the physical resources of adults, so when they get
into trouble, things go south surprisingly fast, especially around
water. An amount of water that we could just cough up can drown a baby
very quickly. Think about how small a child's lungs are - they're tiny!
Even a five or six year old has a very low capacity for processing
water in their lungs. So it's not about whether or not they could climb
out, it's about whether they accidentally drew in a big breath if they
were to tip over or fall into a body of water.

Water has the dubious honor of being one of the easiest ways for a
child to die, and the easiest mistake to make. Luckily, it's also an
easy mistake to prevent.

Your suggestions of a fence or a grid are steps in the right direction.
The author's concern is not hysteria, however, it's just caution.