"Cultivating an interest in any subject or discipline is more a part of chance than necessity (what we would like to do for our children)." This is what my dad says anyway. I began my own studies by reading through an old edition (1987) of World Book Encyclopedia. It's really fun even now! My dad picked up the entire set for $10.00 at some thrift shop. That was third grade. I was nine-years-old. He was pretty proud of that buy. But I remember turning to the philosophy section (quite by chance), and here my dad and I had the longest conversation I can remember about anything. He's really gifted. He makes everything exciting. Almost a life-and-death situation. But, he knows, and teaches my little sisters and I not to take things too seriously.
Well, to answer your question, my dad recommends children's poetry and the Classics Comics (yet around if you can find a few; maybe 'e-bay'). Also, he says the classical Greeks were first interested in poetry; myth and the theogonies preceded the pre-Socratics. Then came philosophy. Drew Hyland says the same thing in his book, The Origins of Philosophy (1973). He's also a big proponent of myth as a source for interest in philosophy. Over time, see to it that your nine-year-old reads the Iliad and the Odyssey (this is probably best at age eleven or twelve if he is in a G&T program somewhere). At nine-years-of-age, perhaps movies are better. I rembember the movie Troy with Brad Pit. I was really young; but, what a great movie! It inspired me to read Homer.
Also, there's art. My day tells me and my little sisters of his college days and how he took an art history course by chance. The world of philosophy opened up to him in the study of art and architecture. There's tonnes of stuff on that.
Better, if your's is a Christian family, there are stories about our Juedeo-Christian heritage. The Holy Bible Picture Book or The Living Bible for Children are really great! It would be especially exciting if your child could memorize whole passages from the KJV conjunctive with The Children's Bible. My dad says, "...this is good advice for ages five and up." Grandma, too, reads these. She is genuine about this. The children's section at your local Christian book dealer or Barnes & Nobel is a good place to begin. Or, better, the public library. We go there most every day, but not weekends.
Basically, pick up anything. You never know what might be of influence. But, my mom says: "Buy for the child's interest." My dad agrees: "...first, know your child; never stop listening; listen well." But, of course, as you know, don't be forceful.
My dad asked me to tell you, "...buy an old empty bookshelf; put it in the child's room; over time, just fill it up; it will take more than one book or movie; ...it's a life-time's work" Whew! At least until the kid is seventeen, or so. My dad says, I'm "...not there yet."