I agree that finding the deaths of babies and young children is inexpressibly sad, and that we should remember them in our trees.
A recent WDYTYA that I saw shed some light on one reason why a number of children in a family may have died young. Apparently syphilis was extremely common, practically endemic, in the population in past centuries before antibiotics, and could be inherited. The unfortunate person could be born with a disability such as blindness or with no major symptoms, but it would mean that many of their children would fail to thrive and die. The effect apparently wore off eventually, so later children may have survived. A cruel thing to happen! I didn't know all this, and found it quite interesting.
MarieC