It's a good idea to get on with introducing them quickly, as most adult cats will regard kittens as a minor irritation.
(Q) What's the best way to introduce two new kittens to my adult cat?
(A) Behaviourist Jon Bowen says: It was a wise decision to get two kittens rather than one. Adult cats often become the target for unwanted attention and demands for play from kittens, so having two kittens that can play together gives the adult a chance to sit back and watch them rather than become part of the action.
It is a good idea to get on with introducing them quickly, because most adult cats will regard kittens as a minor irritation rather than a threat. This changes quickly as kittens become older, bigger and a greater source of competition.
It's vital to ensure your adult cat has plenty of places to climb and get away from the kittens, as well as freely available food, water and toilet locations. Remember that the population has effectively trebled, which will place greater demands on existing space and resources. Your cat will be far more accepting of incomers if the environment is right.
Using a cage or pen is a good idea for introductions, but only for young kittens. Never try introducing an adult cat into a new home by placing him in a cage for other cats to check him out. This is highly stressful for the caged cat and will not improve the chances of a successful introduction. With kittens, use a children's playpen because this gives kittens plenty of space to move around. Give your cat as much exposure to the kittens in a playpen as you can, but always allow your adult cat to leave the room if he wants to.
At the same time, swap some bedding between the kittens and your adult cat, so that both get exposed to each other's body scent. You can also 'introduce' the scent of the kittens by stroking them and then presenting that hand to your cat when you say hello to him. To begin with he will wrinkle his nose and back away, but quite soon he will ignore the smell or rub against your hand.
Gradually you should find that your adult cat becomes more relaxed being in the room with the kittens, and will sit on your lap, eat or play a game with them around. Then you can start to let them out together, but only when supervised and only for short periods until you can see there is no animosity.