How old is your kitten? You can safely feed your kittens adult cat food for up to 4 months. If you want them to be on kitten food longer than this, then talk to your vet about how long you should continue feeding them.
You will need to make sure that the kitten food you are buying contains enough essential nutrients like taurine and niacin (vitamin B3). You can find these in some brands of adult cat food, but they are harder to find in the form of a liquid supplement. Some online stores sell liquid supplements for cats made especially for kittens by including vital growth hormones. These supplements contain added vitamins and minerals which might not be present in the human versions that you buy at the supermarket or pet shop, so it’s best that you check with your vet before giving any supplemental diet items too much importance.