Website Review
Applied Scholastics
Book Review
Scholastic Dictionary
What I love most about this dictionary is that it has a lot of words, and adequate definitions for them -- but the definitions are not complicated themselves. Most of the words the kids need to look up are in there, and they don't get confused on the definitions, either.
My kids use this one all the time -- we've bought 5 copies to date, and since I have 6 children, we will undoubtedly get more as time goes on.
My friend, who is the head teacher of a K-12 school, has several copies of this dictionary in each classroom, and prefers it to any other simple dictionary.
This is really an excellent choice for children.