Before class, get several sheets of pink and red construction paper and cut them into small shapes that are around 1-centimeter to 1-inch wide. Give each child his own sheet of paper that is a different color from everyone else's. Tell the children to draw a heart, and help them if they cannot draw one on their own. For especially young preschool classes, help them glue a simple line of red or pink paper pieces across the heart. For older, pre-K classes, encourage them to glue the pieces anywhere inside the heart shape.
Many young children love to talk about their families, and depictions of family members are often the first artistic expressions a child makes. Encourage the kids to draw a family member, friend or pet with some crayons. These drawings may be a scribble or a circle as far as you can see, but in the child's mind, it looks just like mom or dad. Have the kids glue small, cut-up pieces of paper on their drawings to decorate clothing, hair, fur or collars.
You can also help kids explore three-dimensional art by letting them create a mosaic on an object such as a cup, bowl or small box. Because they are young and may not understand complex decoration ideas, you can have them paste simple bands or pieces of paper all the way around the item. Encourage them to make a full circle, but praise them for whatever they may do.
A wide variety of items can be substituted for paper, especially if you're working with older preschoolers, but you must be careful of choking hazards for children of all ages. Common items are kernels of corn, seeds, flower petals, craft sticks and cotton balls. Have children glue them together on a sheet of paper to create flowers, trees, pillows, animals, gates and more.