When we first started homeschooling, my little boy was in the 2nd grade, and at that time I voted WordCentral.com as the best online children's dictionary. I am now slightly altering my vote. I still hold that this online kids dictionary is the simplest to use and has the simplest, most concise all-in-one definitions for most words - which is GREAT for young elementary kids. But now my boy is in the 5th grade, and our dictionary needs have changed. My current favorite online kids dictionary is WordSmyth.com. This dictionary lets you chose from 3 different "comprehension" levels - beginner, intermediate, and advanced - beginner being usually as simple and concise as WordCentral, if not more, working up to advanced having a more expanded definition, usually with multiple possible meanings for a word, but always with definitions still easily understood by a child. Plus all levels of this dictionary include more information about the word than WC's entries do, more like you might find in a "grown-up" dictionary, which is great for the older ages. It's a little less streamlined to use, but still easy-peasy for my highly internet-capable 10 yr old. :)
Example definition for "comfort - (v)" from each:
1 : to give strength and hope to : CHEER
2 : to ease the grief or trouble of : CONSOLE
WordSmyth - beginner:
to make someone feel less pain or worry.
WordSmyth - intermediate:
to give relief from a painful or difficult situation; soothe.
WordSmyth - advanced:
to give (someone) relief and strength in trouble or distress.
And there you have it.