I was reading adult and young adult books by the age of 14.
I haven't read this one in particular but Penguin Classics tend to be alright as a pseudo-scholarly introduction to a reader who is otherwise naive about the subject.
I'm pretty sure I read the Penguin Classic edition of the Dhammapada at one stage.
I guess the classic introduction to Buddhism is the story of Gautama's meeting the "four divine messengers" and leaving home, becoming enlightened -- see for example Meeting the Divine Messengers.
I think that story is published in innumerable editions and translations, some wth more and some with less attempt to make the story seem realistic (as opposed to e.g. supernatural, viz the Buddha's remarkable powers and Mara etc). Canonically I think that story is the introduction to the Jataka.
I haven't seen the Gathas in an edition for children, it might be an interesting suggestion though -- perhaps something to read together and discuss?
The commentary to the Dhammapada consists of stories (including the supernatural), one per verse (and many verses), see e.g. Buddhist Stories from the Dhammapada Commentary -- the stories are included in this online edition (click on the links to each verse e.g. Verse 1) I'd guess there might be a for-children edition of that.
A Google search for Buddhist school books might be useful, for example my first result from that search was this page: Buddhist Studies for Schools: eBook Library (PDF documents)