Does anybody know of a book which explains the meanings of words for Children?

[Tuesday83]

I am about to start volunteering with an organisation which helps children to read. I find if a child knows the meaning of a word (rather than just how to say it) it helps them use that particular word more confidently.
However, I am not always great at explaining long words to children - and tend to use other long word definitions, which would in fact confuse them more.
I need to find a way to explain the word in the most simplest of terms!

Does anybody know if there is a book - or even a website - which does this that I can turn to for reference when reading with the kids?

[Count-A-Strong]

Have you thought of a Childrens Dictionary?

[Tuesday83]

I did try a Childrens Dictionary but there are a limit on words sadly which doesn't really help at all.

[alexanderd]

A childrens Thesaurus