Telling such tiny children "not to" won't work - they don't have the inner experience yet to use such information. They are experimenting - kitty does interesting things when they are "mean" to her. And though they can know that it is not ok to hurt kitty, their "meanness" is not introspective, is not arising out of a moral basis - so it can't really be termed "mean".
This is not in any way to say that it is ok for them to hurt the cat! But what it does mean is that you will have to be extra vigilant and make sure that you are right there so that kitty does not get hurt. And it might mean you need to put the cat out more or close her into a room where they can't get at her - if you would not know that they did this. You have to both protect the animal and be preemptive to avoid them hurting her.
If you put the cat out and the children ask for her to come in, you can say something like "Kitty would like to come in - but only if we all have our gentle hands on. Can you show me how you use your gentle hands?" etc. If the "rough hands" start to be used, then kitty must go back out (or into another room) and you need to say something like "Oh dear. Looks like you forgot your gentle hands. Kitty needs to go away now. She doesn't like rough hands."
Good luck - this could take a lot of energy on your part - but in the end it'll be worth it!