That is a really wierd situation. In any case, if her kid is old enough to safely wander around by himself, there is absolutely no reason why he couldn't be left at home. If he is not, there is no excuse for him being ditched to wander around in a shopping mall all day.
There is, of course, the possibility that it is a single mother who can't afford a babysitter, doesn't want to leave her child alone and isn't allowed to have him right next to her while working, who has concluded that this is the best possible solution.
If it were me, I would question how long it has been going on and how much of a problem the child is creating. If he was just a decent little kid who wasn't causing any trouble, I'd talk to him when I wasn't with a customer or busy and just let him stay. Sometimes getting mad about people's possible hard-luck situations only makes them worse.
In any case, as others have said... if he is a problem that can't be dealt with, the best course of action would be to nab Blart next time he rolls by.