My boss told her 9-year-old daughter Becca that school was cancelled because of all the snow. Becca says, "What? There's no snow." And her mom says, "Dude, did you LOOK outside?" and Becca's like, "Yeah, when I let the dog out." And mom says, "You'd better look again." And Becca goes to the window and gets this extremely puzzled look on her face. Once she got the joke she thought it was hilarious.
My favorite April Fool's story involves a four-year-old named Trevor who hated taking baths. His father Keith had been explaining April Fool's Day to him, saying that it's a time to tell silly stories that aren't true, but you shouldn't tell anything really mean like saying someone died. So Keith gets Trevor up in the morning and says, "Trevor, your hair is so dirty we're going to have to wash it seven times." Trevor starts to get really upset, and his dad says, "April Fool's!", and Trevor says, oh so reproachfully, "Dad, that's like saying somebody DIED!"