I believe stores generally have a back door leading to a non-public hallway or directly outdoors for the purpose of deliveries, right? We would duck into a store and try to locate that way out.

Failing that, I would observe the gunman from a place of cover and concealment if available, and try to use my knowledge of firearms to my advantage. You said an SKS. Look for: Does it have an external mag sticking out? If not, it's a standard SKS that uses an internal 10 round magazine that takes a few moments to reload, even with stripper clips. So try to count the gunmans shots to determine when he might be temporarily out of ammo and use that brief period to make our "run for it" through his normal field of fire (if that were the only way out). If you can see an external magazine sticking out, assume that to be either 20 or 30 rounds. And also realize that reloading this setup will be much quicker for the gunman than with stripper clips and an internal mag.

If our family was unable to escape through a back door or by running for it, we would hunker down in a small protected space that would require the gunman to come right to us (up close) to find us. This negates the advantage of his rifle and gives advantage to our handguns. A dressing room door leading to dressing rooms going off to either side would be ideal (watch for mirrors in the doorway that might give away your position). Set up on both sides behind the main dressing room door, CCW handguns drawn and ready (both mine and my wifes) with one kid each crouched at out feet (ready to spring out to tackle someone coming in). My kids are older and able to help in defense. Obviously we would give due consideration to NOT shooting at ourselves in any crossfire!

And there we would wait until hell froze over for the situation to be ended by the police authorities. We would use our cellphones to call 911 and tell the dispatcher our location and defensive setup. If the gunman somehow happened to stuble upon us, we would have as much advantage as possible due to preparing and setting up a defensive position.