No, I would just recommend that it is at the child's reading level, rather than above, since a higher level can be intimidating. Encyclopedia's are inexpensive and it is better to just purchase another one as a child gets older. This encyclopedia I'm speaking of is already narrowed down to a child's science encyclopedia, so my feeling is that any one of them would be a good one. Lots of attractive photos, of course, and a broad variety of topics.

I had several growing up, and really loved them. One of them (my first) I believe was called the "I Wonder Why Encyclopedia". I looked on Amazon for it, and it appears to be a rather popular title. I would be inclined towards just going to a bookstore and investigating their selection.