When my children were small my husband's work took him away from home a lot and I was worried what would happen if anything happened to me in his absence so I arranged with my parents that whenever he was away I would ring them every morning and if I didnt they would ring me. I was also fortunate that DS was born calm and responsible so by the time he was three he knew his name and address by heart and knew that if mummy ever went to sleep and wouldnt wake up he should dial 999 and tell the policeman.
More usefully to us now Age UK has a form that you can fill in with all the information anyone may need if you die or become incapacitated, which you leave somewhere accessible. It can be found at the end of the following document on their site:
www.ageuk.org.uk/Documents/EN-GB/Factsheets/FS27%20Planning%20for%20a%20funeral%20November%202010_fcs.pdf?dtrk=true
BBC Radio 4Extra Scheduling Shake-Up.