Type death.toggle true/false Use false to disable death. You will need to do this every time you load your game. Without true/false it does not say death is off/on, just that it's toggled.. It's safer to just use death.toggle false.
I found this cheat at the guideline page here. Just have a question. Does this cheat apply only for the active family or all sims in the town in case it's set as false?