It's an awful thing to witness and very hard to know how to respond to it. I work in a position of mandatory reporting and it's still hard to do. But it must be done. The best way I think is indirect reporting... as in report it to the appropriate authorities (police, child protection) with evidence if possible. I wouldn't report it to the security officers at a local mall as they would have extremely limited training in addressing child abuse (if they have any at all) and could make matters worse for the child by intervening.