Dear Melissa,
So sorry about your kitty! Brought tears to my eyes just reading about it. Our 2 cats are 14 and 15......
We lost a wonderful kitty (O.C.) when my oldest son was 1.5 yo, but O.C. wasn't living with us when he died--we had just moved from Indiana to Arkansas, and hadn't yet been able to move him down. He was also old, and had cancer. We told Ben (son) that O.C. died and went to heaven, and (years) later that Auntie Lisa (good friend who was watching O.C. for us) buried him in her friends' yard. I'm not sure how much he understood. I don't remember him asking too many questions (it was a while ago, Ben is 19 now), but that may vary from child to child; also because he hadn't seen O.C. for a few weeks, while your daughter saw your cat right up until the end.
There is a nice book by Cynthia Rylant called Cat Heaven (yes, you will cry); also one called The 10th Good thing about Barney (I forget the author). Those may help.
I think it is OK to say that kitty went to heaven, since you believe in God and heaven. I too believe that pets go there! There is a wonderful "story" called The Rainbow Bridge" that talks about how your pets wait in a meadow for you until you are ready to go to heaven, and you cross the rainbow bridge together into heaven. I also think it OK for you to admit you are sad about kitty's death too, and to cry, and to let your daughter see you cry.
If she has known any other creature that has died, you may be able to draw a parallel to that.
Sorry I haven't been much help, I'll keep you in my prayers. Hang in there, and many hugs to you!
K. Z.