T.,
Cigarette smoke can be tough to get rid of, no matter what you do. I have two recommendations, based on my years of employment history in the automotive industry. The first, and easiest, is to go to a local auto parts store (or do an online search/purchase) and buy 2 products manufactured by Duragloss. The first is called 441 Leather Shampoo, the second is called 221 Leather Conditioner. They are to be used in the sequence that I listed them above. The second product is enhanced with a special leather extract, which enhances the natural leather scent, and it lasts for some time. I love the smell of new leather (think of a new car with leather seating, or a visit to a tack shop), and that is the only scent it gives off. It's also safe for your children.
The second, more agressive approach if that doesn't fix the problem, is to use an ozone generator. These devices are commonly used by companies who do fire & water damage restoration, mold remediation, marinas that offer boat detailing and odor elimination, and even some more professionally equipped automotive detailers. Hotels also use them to convert smoking to non-smoking rooms. Ozone is naturally occuring in the environment, and is three oxygen molecules bonded together (03, versus 02 for oxygen). Ozone is a natural deodorizer, and can destroy chemical, smoke, mold and other odors. The scent is what you would smell immediately after a thunderstorm when lightening is present. Some people liken it to the smell of clean sheets. If you've ever used a dryer at a laundromat, you've probably noticed the same smell, as many commercial dryers incorporate ozone generators into their gas-fired clothes driers. What the machines do is take oxygen out of the air, separate the oxygen molecules using an ultra-violet light spectrum, and the molecules exit the machine rebonded in threes, or ozone. Over a period of many hours (sometimes days), in a sealed-off environment, the odor is eliminated. I actually own a small ozone generator, a leftover demo machine from my days of selling to the auto industry. These machines are rated in how many parts per million of ozone that they have the capacity to generate. Mine is an acceptable size to eliminate odors in a small area, like inside of a car that has been smoked in or has been soiled by a child or pet. I have also taken small items that needed scent removal, and thrown them in the car for a time, and that has been effective. However, "throwing" your couch into the minivan or suv is not an option for you. You would need access to a higher-capacity (higher ppm) generator, and place the couch temporarily in a small room that you can seal off for a period of time while running the machine. (An acceptable continuous level of exposure to air with high quantities of ozone present has not been established, particularly for small children, pets, older people, people with lung ailments, etc. So you need to keep a room where you're using an ozone generator sealed off with blankets or towels around the door bottom and openings, and keep people out of that area. Upon completion, you need to allow the ozone time to naturally breakdown back into oxygen before habitating that room. The recommendation is generally 50% of the time that the ozone generator was running. (I.E. If the machine ran for 48 hours, allow 24 hours after the machine was turned off before going into that living space.)
I know, it seems like an awful lot to go through, but that's the only sure-fire way to eliminate the cigarette odor permanently, and not layer your couch with chemicals that won't work or are not good for your children.
Best of luck to you!