I know it's a lot of work keeping up with a baby!
In P1, if you haven't been exercising, I believe the book tells you to refrain from it--you can get pretty tired and "flu-ish" during detox and exercise is pretty taxing under those conditions unless you're used to it. If you've been exercising regularly, the book says you can continue it during Phase 1.
However, exercise is VERY important in Phase 2, IMHO. The old book says that it's encouraged, but not necessary. However, the new book coming out in April puts a huge emphasis on the importance of exercise. I think that's much more realistic.
The truth is, the only way to lose weight is to eat less and move more. Any plan, including SBD, is trying to find ways to make it possible for you to do at least one of those. However, cutting out calories (eating less) can only be done to a certain point. You have to eat to lose, and if you eat too little, you can find yourself feeling deprived and thus going back to your old ways. Exercising (moving more) is MUCH easier and faster in terms of helping you shed extra weight and helps your health in other ways, too! Strength training helps you build muscle, which, in turn, helps you burn more calories (muscle burns far more calories than fat, even when you are at rest!). Cardio helps you "burn fat" and use up calories. They're both important parts of any weight loss or health-enhancing routine (assuming your doctor agrees and you don't have any health issues that would make them unhealthy for you).
When you have a very small amount of weight to lose, you definitely will find it difficult (unfortunately!). Exercise can make it much easier and much less frustrating.
I've learned SO much about exercise here on 3FC! We have several amazing maintainers who know a ton and are willing to share their knowledge with you in the Exercise
and Weight and Resistance Training
forums. You can also find advice on ways to workout at home using fitness videos in the Fitness Videos
forum. There are several things you can do at home, possibly while your baby is napping or playing. When your baby is a bit older, there are even videos meant for mothers and toddlers or mothers and young children! It's a great thing to set a good example for your kid, too.
I know one video series that's very popular is the "Walk Away The Pounds" series (the WATP ones that Schmoo mentioned above). Apparently it's very easy to learn and very effective. You can get a lot of exercise going for walks (especially where there are hills) while pushing your baby in a stroller. A dear friend of mine and I used to go for walks twice a week when I wasn't working. She would push her child in a jogging stroller (but she sometimes used the other one if her husband had the first) and got tons of great exercise!
I hope you can find ways to include exercise in your daily routine! It's so important!