Getting a new kitten when our cat uses the cat flap?

If the kitten is small enough, you can simply put a barrier up near the door that you are able to step over. The grown cat will be able to jump over and continue to use the door flap, but the kitten won't. A cardboard box works well if you cut out one side and place it against the door. The older cat can jump into the box then out the flap, but the kitten will not be able to scale the box as well.