Instead of using your kid to try to "get by with something" you should have just gotten your stuff once, and then gone back through another line with another cashier. (Same result, less potential for hassle - which you ended up getting)

To me, doing what you did would be like having 30 items in your cart and splitting them up into two orders just so you can use the "15 items or less" express lane.

Sure, it doesn't specify somewhere EXACTLY that you can't do that, but it goes against the spirit of the express line, just as your actions went against the spirit of the promotional offer. I figure that's how the people at your K-Mart felt anyway.

You tried, it didn't work... perhaps you could try again at another K-Mart since you have 10 more coupons. You could open up a beauty shop by time you get through all of them. ;-)