Star I'm really sorry about your kitty. Children do have profound memories. I agree you should just tell her.
I speak to my children often about death so they are not so curious about it. We have a lot of fish for pets (not as cuddly as kitties but my children love them just the same, especially when one fish died recently that was as old as my son ), so I have this happen often unfortunately. I just try to be matter of fact about it but try to make it sound as if it is not so much about the "end" but a "beginning".
They have been told that God has created all the living creatures and is always watching over us. I tell them the concept that we will all be in Heaven someday and see each other again and God chooses when we come. God actually decided that he missed the fish and needed him by his side for a very important job, etc. Add to it as needed with their questions.
Good luck. I'm really very very sorry about your kitty.