Well, this is something we don't expect from humans. No human, except maybe high-ranking businessmen, leaders and armchair ethicists ever does that kind of calculation, because in the usual case we meet them, there is no time. When you're a second before a car crash, you don't count the passengers of the other vehicle and evaluate their relative worth compared to yourself. You swerve, or brake, or generally do whatever you can to not hit them. And so will the self-driving cars, except better. In fact, I think we can avoid the moral issue altogether by just focusing on making it nigh-impossible to surprise an autonomous vehicle in such a way that it won't be able to reach a safe state[0] in time. After we have that, then we may consider optimizing the algorithms to take into account that e.g. the other vehicle is empty, so its suitable to be used as a braking device.
[0] - Either avoidance trajectory or a speed that would not be likely to kill a pedestrian if they were hit directly.
Then if you run the outlandish scenario, and assume extraordinary leaps in AI comprehension in the next 30 or 40 years, it still remains silly. How many teenagers are equal to one baby? How many puppies does it take to equal one 80 year old grinch? There is no scenario under which it will work. Ultimately the AI will not be able to make accurate judgments about morality / character, it can never choose correctly even if you try to program it with what would be strictly subjective value systems - that one flaw guarantees it can never work properly.