@pganssle Here are fiction recommendations based on what I enjoyed when I was in that age range. Both are starts to longer series in case they're well received.
Relativistic effects are used in Ender's Game to allow characters to age at different rates, and it was my assigned summer reading for freshman year at my all-boys high school, so that's my recommendation for your nephew.
Sabriel has a competent female protagonist, but her enemies are so powerful that she has to outthink them more than outfight them. I read it in sixth grade, so that's my recommendation for your niece.
For nonfiction you could consider Randall Munroe's books (Thing Explainer, What If?, and How To) for your nephew. I don't have much idea here for your niece, so I'll offer the Golden Compass as a second fiction choice for her.