Answers:

1. Harold and Caleb always do monkey business whenever their parents isn't looking.  
2. He was always reminded that curiosity killed the cat, yet in the end, he ended up doing it.
3. She is so nice that she wouldn't hurt a fly.
4. Adam is such a bookworm, he always go to the library to read many books.
5. They were told to hold your horses until it's safe to cross.

Explanation:

Monkey Business

• A mischievous or deceitful behavior.

Curiosity Killed The Cat

• Being inquisitive about other people's affairs may get you into trouble.

He Wouldn't Hurt a Fly

• Used to emphasize how inoffensive and harmless a person or animal is.

Bookworm

• A person devoted to reading.

Hold Your Horses

• Could also be "wait a moment" or "wait slow down"