First, if you haven't already, you need to get your cat a scratching post that they like. You might have to try a few different types of scratchers; (vertical, horizontal, incline positions; covered in cardboard or sisal rope). Ideally, you should put it near the couch. Then, you can cover the part of the couch that the kittens are scratching with a special double-sided tape like this one: SmartyKat Scratch Not Tape. When you see your cat scratching on their post instead of the couch, reward them with praise and/or a treat.
I second abyeb's advice about getting a scratching post. In addition, one of the best investments I ever made was buying a washable couch cover. You can find relatively inexpensive ones on Amazon.
I bought a cover for my new couch at Target online. I5 have baby blankets over the arms since the cover doesn't totally stay in place. Also baby blankets on top.you can use Kholes,bed bath and beyond, or wall Mart. Hard to return things on Amazon