please dont feel you failed him, its always hard to know what to do in this situation. what most people do these days is pull out their cell phones and video it and post it on facebook. I am glad you did not do that heres what I do when i see a situation that does not sit well with me...

I look around to see if there are any mall security around. (usually they are there nearby dealing with another situation) I go to them and say...

Im not sure whats going on over there but could you please go make sure that child is ok. Where can I find your security office where they can download what I have on my cellphone about this. then I go to the security office tell them what time the situation happened so that they can isolate their security footage of the situation so that they can isolate the footage.

malls always have security camera's and people behind doors monitoring the camera's. most likely what happened was mall security were on their way or calling the police already.

another thing sometimes when this happens in my city it turns out to be one of those how would you react drills. you know like how schools have fire drills and shooter drills. malls do training of their security officers too by activating drills. what you may have witnessed may have been a live training exercise for their employees.

drill or no drill if no store employees reacted and stepped in Im guessing some employees are in some deep trouble for not doing their jobs.

my suggestion is contact your local police department, tell them what day and times you were at the mall, what you witnessed and they can go to the mall, look at the security footage and take appropriate action.