Now would be good? You can probably construct appropriate questions for a five-year-old based on the Eliezer's version of the fundamental question of rationality: "Why do you believe what you believe?". This can apply to a playground fight as much as to a big political issue.
I have attempted this with my daughter. "Is Father Christmas real?" "Yes!" "How do you know?" "Because [best friend] saw him!" "How do you know [best friend] is right?" "'Cos she is!" At this point I had exhausted a 5yo's philosophical introspection.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.