My son was 5 at the time, I woke him up, sat him down in front of the TV, told him this was historic and you will remember it for the rest of your life.
He asked questions, I told him the answers as straight forward as I could.
He remembers it and talks about that day watching TV. A lot of his classmates were censored by their parents. Many did not believe him in his class that day until the teacher started talking about it.