One April Fools Day when my kids were probably 8 and 3 I decided to play an April Fools joke on them.
They were in school and day care respectively and I figured they'd been hearing April Fools jokes all day long. That evening as we sat down at the table for dinner I said "Uh oh. There's a great big spider under the table." Both kids eyes got bigger and they peered cautiously under the table. I said "APRIL FOOL" and laughed. My oldest daughter jumped off her chair, ran and put her arms around me and burst into tears. She said "Mommy you scared me." My other daughter was spooked too. For days after that, every shadow or spot on the floor she would say "Is that a spider? Is THAT a spider?"
It's probably a good thing they weren't in my care for their whole lives. Who knows what might have happened?