Kittens will naturally chase a bird. A bird can die from a scratch from a kitten. Please do not let the kittens near the bird & have them locked up when your bird is outside the cage.
I have birds, cats & dogs in my home and the birds are kept away from both the dogs & the cats for their own safety.
Any time I see a cute picture of a bird with either a dog or a cat, I wonder if that bird is still alive. I personally know of 2 birds that died - one from infection after a dog bite/scratch and the other after a cat scratch.