Gravity:

Gravity basics: From one of NASA's International Space Station experts.

Gravity basics: From an astronomer's point of view.
 

Black Holes:

NASA Kids Black Hole Page: Follow this link for an introduction to black holes from NASA.

Astronomy For Kids: Click on "Click here to visit this site".  Good black hole basics.

The Truth About Black Holes: Follow this to fall into one of the eeriest celestial phenomena.  Look carefully for arrows to follow.