Its worth talking about a 'Living Will' with them - this is a docuemt setting out their wishes as well as practicalities. Talking it through withh them is a good way of having a conversation about what they want to do as they advance in years.
Age UK has useful information - https://www.ageuk.org.uk/information-advice/money-legal/legal-issues/advance-decisions/ and in the local area there may be an advocacy service or older peoples charity that can help prepare this document. Sometimes its easier if someone else, one step removed from the family, is administering the process!
My father died earlier this year and it was really useful as his living will, compiled about 5 years ago, set out his wishes which we used as a guide to how to appraoch his care in the last few months.
He set out things like how much medical help he wanted, whether he wanted to go to a care home, people who were important to him and who he wanted to be contacted if he was admitted to hospital as well as information such as where his will, his birth certificate and other docuemnts were and what bank accounts he held. It also expressed his wishes for the funeral. This was produced as a document and electronic copies were circulated to immediate family so that we all knew of his wishes and practicalities.
As part of the preparations he also set up Power of Attorney. This is really important to do whilst your parents still have mental capacity - for health and for financial affairs. It can take several months to come through so its best to set up long before it is needed. The POA can give you full access to their bank accounts so that you will be able to manage their affairs and ultimately have a say in their health. I activated it when my father was first admitted to hospital and within a few days of activation online I was given full access to the accounts and had bank cards etc which meant that I was able to pay all the bills etc whilst he was in hospital which eased his concerns; it also meant that he could go to the care home that was best for him.
In terms of producing a Power of Attorney you can do it alone but for reassurance a lawyer can advise and do it for you. The lawyer will be able to explain it better too and hopefully ease any concerns your parents have.
And whilst talking to a lawyer it may be worth them looking at their wills again - make sure they still meet their wishes in terms of beneficiaries - people, charities etc. As well it may be worth compiling a list of their wishes for particular possessions - if they have pieces of furniture, jewellery or other specific objects they want to give to particular people; this doesnt need to be part of the will but can be kept alongside it. This can help you to distribute the estate and minimise disputes in the family about who gets what.
Another thing to consider is writing a list of their online accounts if they use the internet what are the passwords to their email/ facebook/ other accounts so that you can access them and close them as appropriate.
Its worth approaching this process gradually as too much can be overwhelming.
Good luck!