One common idiom for kids is the phrase "butterflies in my stomach," which describes the feeling one gets when nervous. Another common idiom is "cat got your tongue," which is a sarcastic way of asking "Why aren't you speaking?"
The idiom "like finding a needle in a haystack" is a way of saying that something one is searching for is nearly impossible to find. When someone feels like a "fish out of water," it means he is uncomfortably out of place in a particular situation. If news is described as coming "straight from the horse's mouth," it means the person involved in the situation gave the information first hand. When a child is "snug as a bug in a rug," he is tucked into bed and cozy. When someone "let the cat out of the bag," it means he gave away a secret.
The idioms "give it a whirl" and "take it for a spin" both mean to give something a try or a test run. If someone has "lost his marbles," it means he is crazy or insane. When someone "stuck his foot in his mouth," he said something wrong or inappropriate. If something happens "once in a blue moon," it means it does not happen often. The idiom "stick your nose into something" means someone is interfering in something that is none of his business.