http://www.amazon.com/Six-Not-So-Easy-Pieces-Relativity-Space-Time/dp/0465023932/ref=sr_1_1
This is something that someone with basic grade-school geometry can understand. (Probably not a seven-year-old, mind.)
For someone that young who is interested in such, I'd suggest the "Six Easy Pieces", which covers the more "normal" part of science in a very entertaining way.
http://www.amazon.com/Six-Not-So-Easy-Pieces-Relativity-Space-Time/dp/0465023932/ref=sr_1_1
A nobel physics prize winner describes how relativity works using high-school geometry. If you know the Pythagorean theorem (a^2 + b^2 = c^2 for right triangles), then you can learn why time runs at different speeds.
It's really an amazing piece of work.
(Six Easy Pieces is also good, but doesn't cover relativity)
Do you understand the Pythagorean Theorem? That is, how to fivure out the length of the third side of a right triangle?
If so, allow the master to unspin your head.
http://www.amazon.com/Six-Not-So-Easy-Pieces-Relativity-Space-Time/dp/0465023932/ref=sr_1_1