If you are using a two metre wide section of board with your stars randomly distributed, I would move the comet maybe 10 cm. at a time. In this case, of course, you don't want to erase the comet as quickly as you draw it, in order to show that the comet isn't as ephemeral as a meteor. When asking the kids about Comet Hyakutake, ask them if it looked like the fuzzy triangle you've draw. On the contrary, that comet looked like a streamer with a bright circular head. Sound is another subject that is often dealt with at this age level.
Space is airless, so there is no way for sound to be transmitted.
A space odyssey are accurate in this respect. But comets aren't in our atmosphere, so we can't hear them. There is empty space between us and the comet. To answer this question for the kids, it's a good idea to help them get their bearings in the solar system. A gymnasium is a good place for this.