People would run for the crying 5yo (some might just run out solo instead) because it's crying. It's natural instinct. Eliciting such a response is why babies cry in the first place.
I saw a video clip on Pinterest just last night of various animals helping others of other species. A dog jumps into water off a dock so it can rescue a cat (which scratched at the dog in the process, before clamboring onto its back). Another put a goldfish back into a pond. A grizzly bear got a crow out of water it was drowning in, then went back to his business. A cape buffalo flips over a turtle who was upside down in their zoo enclosure...
If a bear can get a crow in its mouth just to place it on dry ground, a human can rescue a crying child.
1000 embryos? We feel sad when we see on the news a family that lost their house in a fire, but not about 1000s of say, Syrians whose entire neighborhood was leveled in civil war. Now, if we were there, and heard crying, we might respond differently. Maybe.
The response is an instinctive one, emotional. It has nothing to do with logic, nor reveals a truth about the biological status of unborn babies. The question is a good one, but reflects people's reactions to what they instantly can perceive. Remember, we already know most 'Pro-lifers' couldn't give half a crap about a child growing up in a slum, or 1000s getting killed because 20 years ago Bush wanted to boost VP's Cheney's Halliburtun's bottom line. It's about being reactive, not applying an equal standard to various differing scenarios.