Recommended dictionaries -- Prices are from Amazon.com. The best thing would be to use the 'Look Inside the Book' feature to view the pages. But seeing the books in person would be optimal and these ones are commonly available.
Oxford Illustrated Children's Dictionary Flexi 2010 [Paperback] Oxford Dictionaries
ISBN-13: 978-0199119936 $16 Age 8 and up
Merriam-Webster Children's Dictionary [Hardcover]
DK Publishing (Author) $14 Grades 2-5
Children's Illustrated Dictionary [Hardcover]
DK Publishing (Author) $21 Age 6 and up