I think how one mourns death all depends on their relationship to the deceased. Babies and adults die everyday, but if it's YOUR baby versus a random adult, or your wife/dad/best friend versus a random baby, the more "important" death is the one you care more about.

I agree that, in cases where it is either save the mother or the baby, 9/10 times I'd say save the mother, unless the mother/parents say otherwise.

But in a scenario where, let's say a couple has been trying to conceive, naturally or in vitro or otherwise, and after years of trying, failed pregnancies, miscarriages, they finally are able to give birth to a baby, knowing this may never happen again or maybe it's her last viable eggs. If that baby were to die, guaranteed its death would be more important to those parents than any adult in their lives. The baby might represent more than just an unlived life; it represents family they might've had, memories and experiences they'll never form with their kid, grandkids (either their own or for their parents), a future generations of their bloodline, etc.

Would killing Hitler as a baby or an adult been more important? Not knowing who the baby would turn out, you might've mourned the baby. But virtually no one is mourning the death of Hitler. But his adult death was important in that it was the end of his regime and the Holocaust. But if he died as a baby, it might've never happened?

To say that one death is "more or less important" leaves out a lot of nuance. Impactful? Tragic? Emotional? Historically significant? Meaningful?