Subject: A very simple way to describe time travel?

I have some young cousins (9 and 11) that are very interested in science . . . 
especially physics.  They asked me to explain how people could travel backward 
or forward in time, and if it is possible.  I couldn't give them a good answer 
except for the information below.

Though most physicists agree that time travel--though the stuff of science 
fiction it may be--is impossible . . . there could be some ways that time 
travel MIGHT work THEORETICALLY.  My question (because I have left them 
hanging, for lack of a good answer) is,  "Could a very simple explanation of 
how it MIGHT be possible is that: a) Travel BACKWARD in time would take a feat 
of incredible velocity beyond any known limit?  b) Travel FORWARD in time would 
require travel at an extremely decelerated rate (almost imperceptible to anyone 
observing)?"

Re: A very simple way to describe time travel?