Idioms are a part of everyone's daily spoken vocabulary, so of course children need to learn what idioms mean.
I've written before about a number of excellent resources for understanding idioms, but my husband introduced me to a new one recently. IdiomDictionary.com is a quick and easy way to find explanations and examples for over 5,000 idioms. Explanations aren't terribly in depth, but it gives a basic definition of each idiom.
For a more in-depth explanation of idioms, you can always pick up my favorite children's book on the subject: Scholastic Dictionary Of Idioms. This book includes over 700 idioms with an explanation and discussion of the origin for each one. It would be an excellent addition to your homeschool library.
Thanks for the ideas! We studied idioms briefly when we read Amelia Bedelia. One of our favorite sites was http://www.funbrain.com/idioms/. Here you are given an idiom and have to choose the correct definition (or the other way around) and with each correct answer a piece of your picture gets colored in as a reward. My girls (5 and 8) loved it.