Evolutionary psychology suggests that certain fears, such as a fear of snakes, may be innate in humans because they were beneficial for survival in our evolutionary history. Snakes have been a potential danger for humans for thousands of years, and encountering a snake could mean getting bitten and potentially dying from venom. Therefore, it is reasonable that humans have evolved a fear of snakes as a survival mechanism.
When a child encounters a snake for the first time, her fear response might be triggered by innate, hard-wired mechanisms in the brain that have been passed down through generations. This fear response, often referred to as "preparedness," is thought to have evolved because it helped our ancestors survive in the wild. It is a way of protecting the child from potential harm.
In summary, a child might be afraid of snakes the first time she encounters one because of an innate fear response that has evolved through natural selection as a survival mechanism.