For a toddler, use very concrete language--he died, he's dead, he's not coming back. Over and over again. Eventually they incorporate the language, and the repetition, concrete nature of the words is comforting.
Definitely avoid using any common phrases that apply to your child's daily life when you talk about the death (including boo-boos)...no "going to sleep" or "going by-by" or "went away forever" or that kind of thing.
Children at that age have no real understanding of God, religion, or heaven, so talking about it that way tends not to help. If you want to reinforce the idea of your cat going to heaven, it might help to point to familiar people your dc has never met (grandparents that may have passed) and say things like "Kitty died. Kitty is with Grandma now. Kitty isn't coming back."
There are several lists on amazon with books about pets dying. Talking through one or two of those might help.