My younger son was about 4 or 5 I think... I remember him in the suspenseful bit saying "run, Harry Pottermus!" 
Fairly sure most of his classmates had seen at least the first couple by the end of KS1.
Older son read the first 6 before the last one came out when he was seven and a half. We were at Tumble Tots with a child who read at least the first one when she was 5.
You know your own children better than we do. One of my nephews used to get upset watching Thomas and Friends (they do have a lot of danger and accidents) and one of my sisters didn't introduce her daughter to Tracy Beaker because she didn't want to point out that some people abandon their children.