45 litres in 3 days between a family of 4 broken down is:
15L/day
3.75L/person/day
VIC health recommend between 1.2L/day (kids) and 2.6L/day (adults), but that doesn't take into account warmth and running around being kids.
Based on the VIC health numbers your minimum daily intake for the family is:
master: 1.2L
miss: 1.2 (but soon 1.4L)
ms: 2.1L
mr: 2.6L
So a total of 11.1L day for drinking - therefore 3.9L isn't too bad for washing up for a day.
My concern would be that you may discourage the young guys from drinking enough if you get too fierce on water saving, and that's then a bad habit.
We do about that quantity when we're away swag camping - we often take 1 x 10L off the shelf water container that we've saved and a big 20L drum. Now I've got the trailer I've got to sort out the brackets for the 60L water tank and where we're going to run water to after that it should be easier to manage water.
Do the kids have their own water bottles? I have one that goes everywhere with me, and it's a good way for me to keep my fluids up and I know when I don't have it with me I tend not to drink enough. It's a 750ml bottle, and I know that getting through a couple of those a day is me well on the way to having drunk enough.
Not certain that's the answer/thoughts you were looking for, but there's some random musings for you to think about... It's certainly been interesting for me to read - I also found
this bit that talks about a turnover in adults of 4% of your body weight in water; as I weigh 100kg I turn over 4 litres a day... I think I need to drink more...