A terrarium is a collection of live plants, usually in a glass or plastic container, with a balance of light and air that allows them to grow without needing the usual care house plants need. Small shade and water-loving plants are the best kind for terrariums; check with a garden shop or an online gardening site if you need help finding some. Some specific plants that grow well in this environment are Venus fly traps, ferns, moss, ivy, chamomile, begonias, Cyclamen, African violets, and orchids.
To make your own terrarium, you will need a large glass or plastic jar or a small aquarium or other clear container that has a lid. The lid keeps water vapor from escaping so that the terrarium stays moist. You'll need a vent, though, (poke small holes, if your lid does not have one) to allow some air into the terrarium. The only other thing required to make a terrarium is a small selection of plants.
The terrarium has a moist, rainforest-like environment. You might want to talk with your children about other common climates (such as desert, tropical, polar, and temperate) and the kinds of plant life found in each.