I would reccommend planning in advance some, and setting a date.
Say April first. Sounds a way off but will give you time to start breaking old habits and use up the "junk" in the kitchen and replace it with other helathier things. Even if your family continues to eat meat and dairy, at least they can benefit from whole wheat bread & pasta instead of white, and no-sugar all-fruit jam and all natural peanut butter instead of the sugar-salt filled junk.
Although I just made the change of vegetarian to vegan, about 6 weeks before I started McDougalling, I started shopping differently. We used up the overly processed food in the house (boxed mac & cheese, etc) and started replacing it every time we went shopping with helathier options.
During that time, you can (without even saying anything about it) start getting your family used to eating healthier foods. Then when it is time for you to jump on the McDougall wagon, it will be easy changes for you. Instead of being tempted to eat the macaroni and cheese your kids are having, while fixing a salad for yourself; you can still eat your family meal of spaghetti (whole wheat) with marinara sauce.
As far as sugar goes, don't expect yourself to give it up overnight. Next time you're in the store, pick up some honey (or better yet, agave nectar at the health food store) and get used to using that instead. I made cookies this week with 0 oil and 0 refined sugar (used whole wheat flour, agave nectar, honey, applesauce & natural peanut butter) and while they were still delicious (husband ate almost all of them that night) they were definately a few steps above the zero-nutritional-value cookies I may have made just 3 months ago.
This board,
www.fatfreevegan.com, and a few other sources have been a real eye opener when it comes to the small cooking and shoping habits we do every day/week. Start sauteeing in broth or water or even wine instead of oil. Start making those small changes and you'll soon realize you can still have lots of flavorful foods. Experiment with all the new recipes and techniques, you'll have a lot of fun too
Best of luck,
Alyssa