Dr. Gareth Matthews (1929–2011) taught in the UMass Department of Philosophy from 1969–2005 and was a pioneering scholar in the field of philosophy with children, writing several influential scholarly articles and books on the topic that have been translated into a dozen languages. A major contribution of his work was to show that even very young children were capable of doing philosophy just as well as (and in some ways better than) adults.
Here are some links to materials developed by Gary Matthews which can be used to run PiPS session with children of different ages.