Why is it so difficult to get young children to eat? Wouldn't survival instincts make them want to eat in the first place?

No child chooses to starve to death. However, young children have small stomachs and short attention spans. So when food is abundant, they may not eat what is given to them. Also some parents stress way too much about junior not eating. It turns into a fun game for junior to make his parents scramble about to find an acceptable food, and power struggles ensue

A child's stomach is about the size of its clenched fist; adults try to feed children way too much food at a time. They are much better at naturally calibrating their caloric needs than we give them credit for. They could give a shit about the arbitrary schedule adults impose on their eating.