I saw this a few days ago.
The man should not have hit the kid. He should have walked away or called the police and then walked away.
However, he was assaulted. He had the right to defend himself, but decking a little kid that is not armed is not called for, or acceptable, under any circumstances. With his size difference all he had to do was to stiff arm her to keep her off of him when she attacked him. He has the right to keep her away when she attacked, but not to hit her and knock her out. The size difference makes the action he took to be over the limit of reasonable action.
All that said, what the hell were all those kids doing there, harassing shoppers that were going in and out of the mall? We used to have the same problems around here until the malls and the movie theaters posted rules about unaccompanied minors and got the police to help out. Now, kids have to have a parent with them, or they're not allowed to be on the property.
https://www.shopnorthlake.com/youth-supervision-policy/