Kittens are usually more playful than adult cats. They practice fighting and hunting through playing with each other at this age, so it's possible they're just treating the chickens like another kitten and playing a little rough. They may grow out of it as they get older.
In my experience cats are either hunters or they're not. I've had lots of cats, and though they looked the same, they have very different personalities-the last cat I had would kill just about anything. He would travel off into the wilderness and come back hours later with all sorts of prey-ferrets, ravens, snakes, shrews. The cat I have now (even though he looked just like my old cat) just doesn't have that instinct. He never leaves the yard, for one thing. I've never seen him get into that "stalking" mode with any critter. I've introduced him to my bite-size baby quail and he's perfectly friendly to them, he greets them with a curious sniff and then goes on his way. So it may just depend on the individual cat. Some are big on hunting, some are more content just laying around in the sun. If your kittens don't outgrow it, you might have to find a way to separate them from your chickens.
