Enchanted Learning, Merriam-Webster and Wordsmyth have online dictionaries made especially for kids. These online dictionaries cover a wide range of learning levels from the youngest and most inexperienced learner to the more advanced student.
Very young learners can find help on the "Little Explorers Picture Dictionary" offered by Enchanted Learning where, just as the title suggests, children can see pictures of the words they are learning about. "Word Central" by Merriam-Webster includes a thesaurus, a rhyming dictionary and games. The "Learner's Dictionary," also by Merriam-Webster, includes quizzes, a "Word of the Day" and core vocabulary. Wordsmyth's "Kids.Wordsmyth" includes three different levels of difficulty, quiz makers, glossary makers and puzzle solvers. Most of these dictionaries include sound files for help in pronunciation.