I ran a whole test run. They never hit babies, but would very often hit someone else when the offender was out of sight and reach.
Another bug, though. Since they avoid babies, having an offending female give birth protects her fully, all sins forgiven. After that, the bear starts acting strangely. Usually, they can pursue at least two nichelings and give them all one swipe for their offense. After the nest glitch, however, they would find one nicheling to take punishment for all. Still, almost everyone had a baby killed by a defender