There are many ways of explaining the trinity to children, but the one I like the best is very visual, and involves the use of food coloring.
If you ask at a cake shop, they should be able to tell you where to obtain food coloring in powder form. This comes in a variety of colours.
Stick small pieces of double-sided sticky tape to the inside bottoms of three glasses. Choose three suitable colors, and place a little on each tape. These food colorings are very concentrated, so only very small amounts are required, and are consequently unlikely to be noticed by the children. However, you will probably need to determine by trial and error the best way to do this preparation.
Show a jar of clear water. State There is one God, represented by the water in this jar, who is made up of three different Persons – Father, Son and Holy Spirit. As you are talking, pour some water into your three glasses to produce the three different colors.
Depending on your group, you could then go on to discuss the different aspects of the three Persons of the trinity in more detail.
A similar illustration could be used if you are teaching about the water into wine.
by Maurice Sweetsur, http://objectlessons.blogspot.com/