I was invited to spend some time talking with my son's third grade class at Glenwood Heights elementary school (near Battleground, WA). They are starting a science unit on space. I was apprehensive not knowing exactly how to present the material at a third grade level.
We talked about Galileo and the first telescopes. The telescope as a time machine. The real time meaning of a "light year", the gigantic size of the Milky Way, and the unimaginable distances to the nearest stars, the nearest galaxies.......out to the most distant galaxies. We talked about the concept of the "Visible universe", and what may lay beyond.
I needn't have worried. The kids were attentive, and enthusiastic with tons of good questions. The session lasted one and a half hours and could have gone on far longer. Several of the kids skipped their recess to stay and ask more questions. My son told me that the kids talked about it the rest of the day......some looking forward to seeing "Orion" and the the Orion nebula that night. Some of the kids have telescopes or access to a parent's telescope.
If you have a chance to do this, jump at it. You will be amazed at the level of interest and understanding that kids have. Be prepared for some very complex questions. Example: "Why are the rings of Saturn in a thin, flat plane?"