I take it that "phasering" Eileen will result in a opening through which Kirk and McCoy can escape? But would Eileen and child die in any event through injuries and/or starvation subsequent to Kirk and McCoy's demise? Or would she clearly have the ability to free herself and child, perhaps after regaining consciousness?
If she and child were doomed to death, I would say disintegrate them both, at least accomplishing the salvation of two of the four people involved in the cave in. However, it would be incumbent
upon Kirk and McCoy (as gentlemen) to sacrifice themselves, so as that mother and child could survive, to lead their world to a future of diversity, freedom and democracy.
It would make no sense for all four to die.