I tried the 5 year old explanation on my son today, with a few extra details thrown in to set the scene. I swear I saw stars sparkling in his eyes when I finished. Worked like a charm :).Nice, glad it worked!
One final postscript I forgot to mention on all of this to make it particularly fitting: today is the 79th anniversary of Pluto's discovery.
In honor of the anniversary, I just came from a department event announcing the newly created position of the Clyde Tombaugh endowed chair. Patsy Tombaugh, Clyde's widow, spoke at the event to a packed room. She quoted Clyde in what I thought was a particularly nice sentiment: "How can people not be interested in astronomy? Don't they want to know where they are?"