K.U.
The second she starts going after the cats, get a hold of her, get down on her level, look her right in the face, and very sternly say "No! We do not chase the cats! It's not nice!". Then have her do a time-out in the corner or sitting on a designated "naughty spot" for 2 minutes. Set a timer and if she leaves the area, stick her right back in and start the timer all over again. After the time-out is done, remind her again that she is not to chase after the cats. You may need to do it a number of times but if you are consistent and firm and make it known that you mean business, she should start getting the message. No more explanations - she won't get that for a while.
This is how I have to discipline my 3 yo DD when she gets all "attitudy" and decides not to listen - then the rest of the day she is magically much more cooperative.
It may take one of the cats giving her a swat to finally get it through to her, but I would also hate to see her really get a bad scratch in a place like her eye. And cat bites are really nasty - they can get badly infected and need to be treated with antibiotics.