Dictionaries written for younger children should include short, child friendly definitions of words most children already know as well as an abundant collection of new words. Images may be included and will probably make the book more appealing to younger readers. Example sentences, pronunciations, synonyms and homonyms should be given for each word.
The Dorking Kindersly Illustrated Picture Dictionary is an excellent choice that fulfills all requirements for what should be included in a children's reference book. It includes sections that explain the parts of speech and word beginnings and endings.
Websters New World Children's Dictionary uses medium sized text to give kid friendly definitions to words that most children know, as well as hundreds more to expand the vocabulary. This dictionary includes a 10 page guide to dictionary usage, parts of speech and grammar rules.
For children in middle school, Websters New World Student Dictionary is an exemplary choice. Containing over 50,000 words, this dictionary provides slightly more complex definitions than the above mentioned children's dictionaries, yet the text is appropriate for the children in fifth through eighth grade. Maps, pronunciation guides, and word origins are included.
One of the best dictionaries for homeshoolers in high school is the Thorndike-Barnhart Student Dictionary by Scott Foresman and Company. This dictionary contains over 100,000 words as well as a tutorial, "Teach Yourself" section, puzzles and a section called "Basic Style Rules for Writers" aimed at helping high school students improve their writing skills.