Security is expensive, the better the more the cost. Just do away with the no firearms signs, get free security.
If malls thought it was worth the money, they could get better security and better security guards. But it is not a trivial problem, starting with the almost universal antipathy to such things by local LE agencies.
You would need something like the training program for nuclear security guards. And something like the same caliber of people. there would also need to be a commitment to ongoing training, and proper staffing levels to avoid the problems the nuclear security industry has had.
You are probably talking about people who would need to be paid something like $15 an hour (plus benefits), with perhaps 500 hours of training prior to being put on duty. The training would probably cost something like $20 grand per guard, maybe more.
Add in ongoing training, and supervision, and you can see that it is a very expensive proposition.
To get a full shift of just 3 guards from say 6am to midnight, would require about 12 FT guards plus maybe 3 supervisors at an upfront cost of around $250 grand, and an ongoing cost of maybe $600 grand a year.
Add in attrition and the need to train replacements continually, and you can see why it is prohibitively expensive for even the biggest malls.
putting regular cops there is even more expensive.