I think a lot of young students struggle with the idea that plants are alive, but don't seem to move. Even though students know that plants grow, many of them think of a plant like a tree, which seems stationary. Although not all living things move, students often associate moving with living, and something stationary being alive is confusing.
One way you can show plant movement is just by getting a plant that will "reach" toward sunlight, and keeping it on one side of your classroom (or one area of the outdoors). This plant will obviously look as though it "reaches" toward the sunlight (certain plants are better than others- I'd check with a horticulturalist, botanist, or other plant expert for ideas). Put some sort of mark on the side of the pot where the plant seems to point- maybe even take a picture!- and then turn the plant around and leave it there for some time. Come back, and examine the change. Talk about why a plant would lean towards the sunlight. This might be a great introduction to plants, and what they need to survive.
You can also, of course, look at fast-moving plants, such as the Venus fly trap or the touch-me-not plant. If you can't get these into your classroom, find a video!
Here are some other interesting plants to look at, as well.