Can you react as if you believe Kappa really exist?
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.
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.