Disclaimer: I am an no way affiliated with any of these types of products.
There is at least one book, "I'm dead, now what?" which contains questions about end of life decisions and spaces to answer. Some of the questions provide some comic relief, making this somewhat awkward conversation a little bit easier to deal with. The books are quite thorough and cover topics like life support, who gets what, preexisting burial plans, and more.
Now that they are starting this conversation with you, I would either get a pair of books like this for your parents as a gift, or suggest to them that they look at them.
You do not need to be there when they fill them out, but you should know where to find them in the event that the books are needed.