Here are some links to resources that you might find helpful with your writing.
Online Dictionary/Thesaurus
Merriam-Webster Online
Dictionary.com
Cambridge Dictionaries Online
Oxford Dictionaries
Thesaurus.com
Online Dictionary/Thesaurus for Kids
American Heritage Dictionary of the English Language (via Yahoo kids)
Merriam-Webster Word Central
Wordsmyth - Dictionary and Thesaurus for Kids
Thesaurus.com - Kid Synonyms
Yourdictionary.com - Kids Dictionary
Yourdictionary.com - Kids Thesaurus
Fact Monster - Dictionary
Reference Library from Yahoo Kids
RhymeZone