Involving a child of that age in real time tank maintenance will grab their interest sooner than any book. Most kids that age are still learning to read, and they are more likely to absorb more in practise than in theory (even where pictures are involved).
My (just) 7y/o daughter is a fish fanatic. Ok, she knows very little about fish we don't keep (though she recognises some species from pictures or visits to the LFS) but she knows about the ones we do keep. I introduce a little bit more to her each time I think she has grasped the last concept...
I take her to the LFS, she names the fish as she moves around the tanks, and has also spotted some signs of stress (lol) and pointedly told one LFS he had a bad case of Ich in his tank of clowns, also advising clowns were prone to it. She told another that the barbs were nipping the fins of a Betta. We don't keep barbs, but she has learned they can be fin nippers, even though they were not attacking the Betta at that time.
If she sees a fish she fancies, she has gone from asking if they are "warm or cold water" to asking if they are fresh or salt, and do they need to be kept in numbers or alone? She is learning about compatibility, and the importance of partial water changes (but I am yet to get to water chemistry with her) She recognises male from female in a lot of species, even those we don't keep, and knows which are live bearers and which lay eggs from a few of the commonly kept species.
I believe that the age group you are thinking of respond much better to practice than theory, and the shared interest with an adult is way more valuable than reading an encyclopaedia of facts.
Bake biscuits with your kid, they will love the shared experience and savour the taste. Give them the recipe and leave them to it, they are in a two dimensional situation with too many questions, and could get their fingers burned.
Involve the child, it is easier (and more fun!) to educate them this way than giving them a book.

That said, my girl does like to leaf through Practical Fishkeeping, looking at the pics!