Kappa
noun
A river juvenile... not what you think. It's a kappa, a mythological Japanese creature. That being said, it lives in / around rivers and is about the size of a juvenile, so maybe this makes sense.
かっぱ
The reading for 河 makes sense. The reading for 童 however is weird. Since the meaning is "kappa" though you can use that to remember the reading as well. If you don't know what a kappa is do some research on the internets and figure out about them. It's interesting, and it will help you to remember this word.
まるで河童が存在していることを信じているかのようにリアクションしてもらえますか？
Can you react as if you believe Kappa really exist?