Collage refers simply to the process of assembling disparate objects together into a single composition. For preschoolers, both the process and the result can be less about art per se and more about simple exploration and fine motor development. That's fine. Use the opportunity to teach about shapes, colors, textures and other aspects of the world around them. Sticky fingers were never quite so educational before.
Collect items of a single color for a color collage and let your preschoolers loose with the glue. Use a variety of textures and shapes for the best results. You can cut pictures out of magazines; use fabric scraps and yarn; provide various kinds of paper, including tissue paper and construction paper; and even include larger three-dimensional objects such as buttons and beads. Throw some crayons or markers of the appropriate color into the mix too if you like. Another fun option with this sort of collage is to have kids paint a piece of paper one specific color and then press objects of the same color into the paint before it dries.
Let your preschoolers enhance their tactile senses with a texture collage. Set out squares of fabric such as burlap, velvet, terrycloth and fun fur as well as similar-sized squares of foil, felt, sandpaper, tissue paper and plastic film. Carpet samples, corrugated cardboard with the corrugations exposed, kitchen tile samples and bubble wrap are some of the more unusual additions you can make to this idea.
A circle is a circle, big or small, black or white, smooth or fuzzy. Same goes for squares, triangles, rectangles and ovals. A good way to reinforce this lesson is through a shape collage. Set out a variety of objects of different textures, colors and sizes, but all of the same shape. For example, for a circle collage you could set out buttons, cotton balls or pompoms, circles cut from paper and fabric and wagon wheel pasta. For a square collage you could use mosaic tiles, tissue paper squares and square sponges.
This idea incorporates some outdoor exploration time into your art project. Take kids on a nature walk and collect items that could be used in a collage. Bring them back inside, set them out on the table with glue and paper, and let the children have at it. Anything from leaves, petals and sticks to small rocks, abandoned snail shells, seed pods, pine needles, feathers and bark can be incorporated into the final composition.
You can have kids put together a collage that fits in with any curriculum theme. For the alphabet, cut out display letters and set them out along with objects that start with that letter -- peanuts and popcorn and purple anything for the letter P, for example. Kids can then glue these items onto a giant cutout P silhouette. You can make number collages similarly, with groups of three objects decorating a large cutout 3. If you have an underwater life curriculum theme, have children create a collage on a cutout fish shape. For Valentine's Day, create a collage from heart-shaped pieces of paper -- it can be fun to use old pages from books and magazines here.