This is exactly the reason why I think "death astrology" is a bad business. If you really look into it, you will find multiple predictive techniques...that contradict one another. Hopefully astrology isn't just a means of feeling more superstitious about things that make us feel anxious already.
Ixaee, if we do the numbers, there is a 1/12 chance that any given person will have the ruler of the 8th house in the 5th. This amounts to 1/12 of approximately 7 billion people on the planet just now, or roughly 580 million people. If all of them had children dying prematurely, I think the life insurance industry, actuarial scientists, and the health care professionals would be all over this one by now.
Note also that the 5th house rules more than just children. It concerns amusements, recreation, and leisure activities of various sorts. It is also the house of love affairs in the sense of something for fun, vs. marriage and long-term partnerships, which are a 7th house matter. It is nicknamed "the party house" or the "fun house." Surely we can think of all kinds of combinations involving the ruler of the 8th in the 5th that do not involve the premature death of a child.
But yes, they will grow up and die eventually.
Dreaming the Seas, actually Mars is the ruler of my 8th house and is in the 5th. Of course, like any mother, I worry that a child might die prematurely. But just now my son is 32 and my daughter is 27.