Caffeine is very bad for cats and dogs. Don't let your cats have any coffee, soda, tea, etc.
One tiny dose won't kill them, but you could be racking up organ damage each time. The HCCUA health website states that cats will get heart and nervous system damage from caffeine. I imagine it would also hurt the liver, kidneys, and GI tract as well.
Plants make caffeine to kill off any creatures eating the plants. Alkaloids (caffeine is an alkaloid) are intended to kill off anything that eats them, not just insects. Humans and other plant-eating animals have developed defenses to common alkaloids, which is why people aren't really affected by caffeine. Cats and dogs have no such defenses.