Pull this prank on a grownup, not a little kid. But make sure you choose a grownup with a sense of humor.
First, fill an empty shampoo bottle most of the way with ketchup. Then add some baking soda—at least half a cup. Shake it up, put the lid back on securely, and wait for your victim to take a shower. When they open the bottle, it will spray all over the shower (mixing ketchup and baking soda causes a reaction because of the vinegar in the ketchup). Thanks to Household Hacker for this idea.