I wrote a post about relaxation/meditation on the corporate website 
– www.rouanzoin.com- Relaxation and Meditation and then I started thinking about the kids and how to teach them relaxation skills.
I do consider a child’s ability to self-soothe, relax, and calm their own mind to be a necessity. In a therapy environment- handling a child’s clinical anxiety or trauma response involves a bit more skill than can be learned in a single book or tape. Living a balanced life (homework/play/exercise/friends/family/good nutrition) is also extremely important in how a child handles stress.
That being said, when I am asked to recommend resources for parents, therapists, or teachers- I almost always recommend the series of stories by Maureen Garth(below). They are meant to be relaxing stories read out loud to children by a parent or caregiver, and children like them. I have recommended them for years- even while working in a children’s oncology unit. Some are even published in Spanish. 
I do not have any current CD’s I recommend for children. I know many families have favorite calming music they use to help soothe their kids at night or on trips etc. If you are looking for a children’s relaxation CD or guided imagery for children, you can search Amazon and preview some of their selections Children’s Meditations
Below are the books available at Amazon or at your local bookseller.