South Beach is a healthy lifestyle that is not only good for losing weight, it's healthy for a family. The focus on whole foods and healthy fats and carbs is really good for everyone in the family. Have you read the book yet? It really helps you understand how it works.
Starting after the 4th actually sounds realistic. It also gives you time to get the cupboards cleaned out and supplied with the foods that will make Phase 1 easier. 40 lbs in 3 months might be a little high. Generally 1-2 lbs/wk is realistic so even with a big Phase 1 loss that's a lot.
There are lots of great tips and recipes on this forum. Take some of your prep time to look them over. The more ready you are the easier the transition will be
Some tips and answers to the most common Phase 1 questions:
Prepare Yourself to Start South Beach
Emergency Handbook for Phase 1
Breakfast Alternatives to Eggs
Phase 1 Time Limit
Baby steps, losing the holiday excess first.
Maintaining 48 lb loss since 2008, working off regain to a sensible maintenance level. 60+ lbs and counting!