dead babies dont cause tantrum spirals, because they cannot socialize and gain friends. They will cause a SINGLE unhappy thought in direct relatives (mom and dad, not sure about brother and siste), BUT its not as big of an unhappy thought as losing a pet, or spouse, or close friend.
edit: checked the wiki.
Lost a (friend/pet/family member) [to tragedy] recently. -50/-30/-20/-10
a -50, is a pretty big hit to happiness, but its to very few dwarves. (babies dont make friends). But at the same time, military dwarves tend to get used to death. End result, dont be afraid to let your mother dwarf take the babies into battle to use as bludgeoning tools in the fight =P, just make sure the CHILD dwarves are stuck inside, in a burrow, so they cant follow mom out to get killed.