There are few books about etymology that are appropriate for adults let alone 8 year-olds. You might want to get a good unabridged dictionary (e.g., Webster's 3rd, American-Heritage) and point out the etymologies for almost all entries.

There is a Ten-Minute History of English available on YouTube (link), but it might be more appropriate for teens.

I'll take a look at my linguistics library and see if I can find something ... you might think about getting a book geared to adults and reading it out loud to your child and trying to answer all his questions.
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Ceci n'est pas un seing.