Some examples of idioms for children are: give it a shot, a piece of cake, cross your fingers, have a shot at, hold your horses, raining cats and dogs, and pig out. These examples of idioms are for children because their figurative meaning is easy for the children to understand.
Idioms are meant to be taken figuratively and not as the literal meaning of the actual words being said. The figurative meaning behind each of the idioms are as follows: to give it a shot means to try, to cross your fingers means to ask for good luck, to have a shot at means to have a chance, a piece of cake means that something is easy, hold your horses means to wait a minute, it's raining cats and dogs means that it is raining very hard, and to pig out means to eat a lot.
There are also many other idioms that children can easily understand and identify with. Some more examples of idioms that children may hear frequently are; to have second thoughts, which means to have doubts, a little birdie told me, which means that a secret has been told, and cry crocodile tears, which means to fake being upset.