I brought with me some samples; the ONU Geology program has lots of samples of rocks & sands (obviously), so I took some especially relevant ones to show the kids.
The main point I tried to get across to them is this: different kinds of sand come from different kinds of rocks. I figured for first graders that wasn't a bad place to start. The idea is to have them connect in their minds that rocks, when eroded, will form sand, and that there is a direct connection between these two kinds of materials. This is, really, their first introduction to the rock cycle.
After I had their interest, I then showed them three other sands and three related rocks. The white sand here is loaded with calcareous material, and the white "rocks" are pieces of some kind of coral from the same beach.
I talked about the three different rocks as representing the three major rock types: the dunite as a metamorphic rock, the basalt as igneous, & the corals as sedimentary. They didn't quite pick up on the differences or the words well (and I didn't expect them to), but they were at least exposed to the terms. They liked the basalt the best - it is a pahoehoe sample from Hawaii, so we talked about lava & how it is a hot, liquid rock that cooled to form this solid material. They were really impressed with that!
Granted the olivine rich sand didn't come from the erosion of dunite, but the samples allowed them to see that there are connections between rocks and sediments.
It was a really fun experience. These students are considerably younger than the ones I'm used to teaching! And, if I'm totally honest, they are in general a lot more enthusiastic about learning than some college students! :-)