Dictionary recommendation?

Rhys is starting to read books with words he doesn't know -- "bolted" was today's -- and I'd like to teach him "looking in the dictionary" as one approach to finding out what a word means. Anyone have a good recommendation for a kids' dictionary, aimed at a 3nd grade- 5th grade reading level or so? (No, that's not the level Rhys is reading at, I'd like this to last for a little while. He's reading level 3 readers, which is I think about a 2nd grade level?)

Thanks.