Question: How can I keep a kitten out of the adult cat food and vice versa?
Answer: You are correct that kittens and adult cats have different nutritional needs, and that it is important that kittens get the nourishment specifically found in kitten food.
A new kitten should be separated for awhile with its own food dish and litter box. You may find that by the time you're ready to introduce the newcomer to your existing cat, they'll each gravitate to their own food dish.
If not, you might want to put the older cat's dish in a high place so the youngster can't get to it. If the older cat wants to sneak a bite of kitten food now and then, it certainly won't hurt her (just be sure the kitten gets its own daily requirements.) The extra exercise the older cat will get by playing with the kitten should keep excess pounds off.
With canned food (which is really better for all ages than dry food), you may have to either feed the two in separate rooms, or feed the kitten first, with the older cat confined in another room. After 20 minutes, take up the kitten food and discard it. Then, switch places and feed the adult cat, also discarding the remainder after 20 minutes.
Another option is to use a premium food formulated for all ages. Felidae is an excellent choice in this category.
There's another thought that you should consider: feeding a raw diet. Many people who are concerned about the quality of commercially prepared pet foods are now taking this route.
The benefit in cases like yours is that the same raw diet can be fed to both kittens and adult cats. Kittens seem to take to it instantly, although adult cats should be "weaned" to it slowly, by adding increasing amounts of raw to their regular foods until they are wholly on raw foods. I wrote an article on raw diets that will give you more information on it.
I also did product reviews on supplements for raw food diets.
As with any other dietary changes, raw diets should be introduced gradually, to preclude digestive upsets.