If your grandson shows interest, one way to start out with Arduino could be to go to one of the many tutorial web sites.http://tronixstuff.wordpress.com/
is one of many, and one of my favorit places to learn new Arduino stuff.
It starts out with very baic stuff but have enough lessons to learn for a long time.
make sure you have all the needed stuff and make sure you start with the abolute basic stuff.
I think it is totally cool and generally ultra nice to see a grandparent wanting to do what you plan!