Why do children get so upset over such small things?
Because the thing is not small to them, and because the equipment for managing the feeling is not built yet.
The developmental part. Emotional regulation depends heavily on the prefrontal cortex, which is among the last brain regions to mature — development continues into the mid-twenties. In a young child, the systems that generate emotion are running at full capacity while the systems that moderate it are barely online. An adult feels the same surge of frustration and dampens it automatically. A four-year-old feels it and has nothing to dampen it with.
The perspective part. Adults judge scale against a lifetime of experience. A child has no such reference. The banana breaking in half genuinely is, in their frame, a significant loss of something they wanted intact — and they have no way to know that in a week it will not matter, because they have not yet lived that week.
The load part. Big reactions cluster around predictable states: tired, hungry, over-stimulated, ill, or coming off a long period of holding it together. The trigger you can see is rarely the cause; it is the last item on a pile.
What tends to help:
Regulate before reasoning. A child in full distress cannot access logic. Calm first, explain later — often much later.
Name the feeling without endorsing the demand. "You are really disappointed about the banana" acknowledges the emotion while the answer stays no.
Stay steady. Your calm is what they borrow to regulate; matching their intensity removes the thing they need.
Do not try to talk them out of the feeling. "It's not a big deal" tells a child their internal experience is wrong, which tends to prolong it.
Frequency and intensity decline as the regulatory machinery develops.