Hi I'm a state registered dietitian. The best supplement is probably vitamin D3 tablets available from Holland and Barrett. They contain no calcium which is probably a plus because adding calcium bulks out the tablet and tends to cause constipation. Vit D3 alone is much better tolerated. I don't think it matters when in the day you take it. Otherwise a lot of the elderly are quite happy to eat oily fish. A kipper has 3 days' worth of vit D requirements. Mackerel is good too and a few breakfast cereals (mostly the Kelloggs ones) are enriched with vit D now. Go out in the sun (assuming we get any this year) between April and October. At other times of year there's no point, the wavelength is such your skin won't be able to make vit D.
Please make sure that your elderly parents are getting Vitamin D3 supplements. Very very important, especially if they are housebound, it can make an incredible difference to their wellbeing as they (and you and your children) will not be getting any between the months of September and March (a bit from oily fish but it comes mostly from the sun). I am a registered nurse and I have been researching this (for personal reasons) and will put an article on Mumsnet somewhere shortly. (Not decided where yet!) Vit D3 increases your immunity especially to colds and flu, reduces your inflammatory response (arthritis and rheumatic pain), shortens infections (if you get any when you start the D3!) Very important for Asthmatic children and adults with respiratory conditions, reduces the risk of heart disease and cancer (especially breast cancer)and diabetes in the young and can alleviate depression especially SAD (Seasonal Affective Disorder). If you don't believe me, google the research and you will find it. Here is one link to Radio 4's "Inside Health" "Listen Again", where the amazing Dr Adrian Martineau (Clinical Senior Lecturer in Respiratory Infection and Immunity) is talking about this. He has been instrumental in getting the government to issue Vit D3 to all pregnant mothers as from February 2012 because it is so important. www.bbc.co.uk/programmes/b01mdfc0