Creating a pyramid to help illustrate the idea of the Holy Trinity can be an effective way of teaching children this abstract concept. Have, or help, kids draw and then cut a piece of construction paper or poster board into a shape that has a square in the center, and a triangle attached to each side of the square. The shape should look like a pointed cross. On one triangle write "The Father" with a picture of a throne, on another write "The Son" with a cross, and on a third write "The Holy Spirit" with a dove. On the fourth and final triangle, write out the verse from Matthew 28:19 that describes the Holy Trinity. Once kids are done drawing and writing, have them fold and crease each triangle into the square so the points of the triangle are pointed inward. Hold two of the triangles together and glue or tape the seams. Repeat on all four sides to create a pyramid.
Hard-boiled eggs are a good way to demonstrate the concept of the Holy Trinity to Sunday school students, especially if you are teaching around Easter. Boil enough eggs for each student in the class. Allow children to use egg coloring to dye the outside of the eggs, or to decorate them with crayons and stickers. Once all of the students have finished decorating the eggs take a serrated knife and cut an egg in half. Ask the children to tell you what parts they see, waiting for them to point out the shell, yolk and egg white. Use the egg as a metaphor to explain that the Holy Trinity, like the egg, has three parts that form one whole, just as the Trinity has three parts that form one God.
Use a three-leaf clover to show Sunday school students how the three leaves are distinct parts that make up a whole clover. Take students to a nearby field, encouraging each child to find her own three-leaf clover. Have the kids sit and listen to the story of the Holy Trinity and show them how each leaf corresponds with a part of the Holy Spirit. Explain that the leaves don't stand alone; they must all be together to form a clover. Have the students pull one leaf off a clover to demonstrate how the Holy Spirit is incomplete without all three parts.
Kids always enjoy treasure hunts, as they can to get up and explore during class time. Have students hunt around the room or in an outdoor space for triangle shapes, clovers or other symbols that the feel could represent the Trinity. After each child has collected at least one item, have them share them with the class and explain how they relate to the Holy Trinity.