You could do worse than to try The Pyn King, a timeless tale of kings, queens, despots and cannibalistic child assassins. For those unsure (and in the uncertain world of self publication who can blame them?) on immediately shelling out for the physical book, you can download a complete digital copy from lulu.com for free.
As I was reading Harry Harrison at ten (Prime Number & Deathworld 1-3), I don't think you can classify a persons reading ability by age. Me personally I'd either get together with them directly and take them shopping, or if that's not possible talk to them see what they like specifically. Fantasy is too broad a subject.