I’d like to recommend a couple of books for 10-12 year olds. They were a couple that I really enjoyed when I first read them and continue to read even now.
Redwork by Michael Bedard is the first book I remember feeling a sense of accomplishment after reading. This is not to say it was a difficult book. When I finished I felt as though I had worked through the story alongside the main character and that I had learned something. The story focuses on Cass, a 15-year-old forced into more grown-up responsibilities than he ought to have by the circumstances of his life. He and his mother move into an upper flat in a large and run-down house owned by the very elderly and eccentric Mr. Magnus. Cass and Mr. Magnus, it turns out, have many more similarities than it would seem at first glance.
The Gauntlet by Ronald Welch (you can actually read the first couple of chapters at that Google Books link). This was probably my first exposure to time travel in literary form (having watched Star Trek for years it wasn’t an entirely new concept). This is another coming-of-age story (a genre I no longer enjoy reading, having already come of age) but it’s set in Wales rather than Toronto (the differences are subtle but present). Young Peter is transported back in time to sometime in the 1300s where he meets several of his distant ancestors and learns about chivalry and knights and jousting and things.
This last book is perhaps better suited to people a bit older than the first two. It’s the first science fiction book I recall reading. Gladiator At Law is a collaborative effort between Frederick Pohl and C.M. Kornbluth. My dad recommended it to me one summer when we were on holiday. The campground/RV park where we were staying had a community bookshelf system where one could take away a book to read (permanently, if you so chose) or drop one off for others. It’s set in a dystopian future where corporations run the entire world and if you’re unemployed, you are basically out of luck. This lowest caste of society is entertained in a bread-and-circuses sort of environment by ultra-violent gladiatorial spectacle. The book looks at a possible future based on multinationals gaining control over every aspect of society. It’s got some pretty heavy content if you really get into it but it’s also a really good action/adventure story on the surface. I tend to blame this book for getting me started on a more or less constant diet of sci-fi for the last 17 years or so.