!! Fear of the dark
It is noteworthy, almost to the point of being cliché, that young children are much more afraid of the dark than adults. And that the reaction to such fear is a desire to crawl into bed with their parents.
There is probably a reason for this, linked to our evolution on the African plains. Leopards like to hunt on moonless nights, so that they can sneak up on their prey. They are masters of stealth; the pads on their feet are so sensitive that they can walk through even the driest, crunchiest grass without making a sound. Leopards may be sneaky, but they're not stupid; even if they can pounce on their prey in total surprise, they still think twice about attacking something as large as a fully grown human adult, particularly if said human adult is in a group. But a small child, left alone by itself, is another matter. Thus, any child that did ''not'' instinctively seek out adult caretakers while in the dark might not live long enough to grow up and have children of its own.
!! Fear of sudden loud noises
Psychological experiments have confirmed that there are two, and ''only'' two, fears that all humans are instinctively born with. They are the fear of falling, and the fear of sudden loud noises. Both of these will cause an immediate, involuntary reaction.