After viewing the video clip, my tentative conclusion is Ow May Chen, mother of the problematic son, is misrepresenting what actually transpired.
1) "Dexter is naive and defenseless" - That's BS. The boy was very belligerent and combative as can be seen from his repeated attempts to disturb the guy's much smaller kid, ramming the guy and at one point using his fists to punch him from behind.
2) Ow May Chen then described the guy as having "kicked, pushed, rough handled" and "physically assault" his son. I can accept that the guy sort of rough handled her son, but she's making it sound as if the guy was raining kicks and punches on her helpless son lying on the ground when in reality the guy simply lost his temper and was somewhat rough in chasing her son away from the perimeter of his much smaller kid who was being disturbed.
We can fault the guy to a certain extent for losing his temper over a misbehaving 5 year old boy, but the bulk of the blame should fall on the boy and his missing in action irresponsible parents. If the boy is normal, then his parents are obviously very bad at instilling basic discipline and courtesy to a 5 year old. If the boy has certain psychological conditions causing him to behave this way, I have to question what on earth his parents were thinking by leaving him alone in a public playing space.