They might just surprise you. The need to keep them entertained is important. Sometimes children who read a lot when younger lose interest and become bored if they're continually expected to read below their level of understanding. Don't be afraid to encourage them to read adults novels and biographies. Often, advanced readers are ready for these well before they reach ten, eleven or twelve years of age.
Classics are a good place to start.
They'll appreciate the plots and themes in adult novels and feel more at ease. Don't worry too much about the content. Advanced readers are mature in their outlook and will learn more if you allow them to choose titles. Although, you might want to have a few personal guidelines, according to your own beliefs. Ensure you make this clear to your kids.