Show the children paintings done by Pablo Picasso. This will give them a good idea of what the genre of cubist art looked like. You might also want to show paintings from a few other cubist artists, such as Chagall or Braque, to help ingrain the concepts.
Talk about cubist concepts while you show the paintings. Giving the children concrete examples while you teach will help them understand the concepts better. For example, you can point out the geometric shapes that are often used to create images of people in Picasso's body of work.
Set up an object that the children can all see. For example, you might set up a bowl of fruit or even a person as a type of still life model.
Allow the children to paint or draw the still life image you set up, but ask that they do so in the style of Picasso, using more abstract and geometric images and creating their own interpretation of the still life. Once the children put the practices of cubism to work, they will understand it even better.