Ghosts - Children secretly hope that ghosts are real so they can prove to their parents that all those nights spent in their bed weren't that pointless after all.
Shadows -
Pretty much every child has tried to run from their own shadow, only to realize it was literally mission impossible.
The hallway - Raise your hand if you've ever sprinted across the hallway to the safety of your bed after turning the lights off.
Bridges - Every time you crossed a bridge, you always wondered if today was the day the infrastructure would collapse.
The "bad guys" - Parents might be at fault here. Kids are told the "bad guys" are going to come get them so often when they misbehave, they actually develop this irrational fear.
Clowns - This fear is often carried into adulthood as well, and movie portrayals of scary clowns don't help either. If you were scared of clowns as a kid, you probably don't want to talk about 'It.'
Grandfather clock - If you ever owned one of these old-looking clocks, you know the panic was real when its midnight ring echoed through the whole house.
Full moon - Full moons obviously meant werewolves, so you knew better than to step outside during those nights.
Thunderstorms - Not all irrational fears are linked to non-existent things. Sometimes nature can be scary too!
Nightmares - Children's nightmares are the stuff of horror movies, and there's no chance they're going back to sleep unless it's in mommy and daddy's bed.