Not that I'm that much into conjecture, but what if it is between your wife and someone else's child? I'm thinking that the better criteria to base a decision like that on shouldn't be based on relations. Either the kid is important to this world, or they're not. Being related to them ought not be a decision making criteria.

Conversely, I would be inclined to put my effort towards saving the one most likely to survive. If it's my child, so much the better, but if it is the neighbor's kid, then that has to be the one to invest the effort in. Sometimes impartiality is a real burden.