Can you take out life insurance on your parents without them knowing?

Possibly, but it would be almost impossible. Since the insurance would be awarded if they died, they would probably be required to get a physical or some sort of test so the company can decide how much the deductible should be etc. Furthermore they would have to sign papers saying they do this in good faith and are sound of mind and stuff like that. If you could somehow get them to do all that and then make them forget that ever happened, it would (sort of) work.

But don't kill your parents