I agree with Mel.
You have to realize though, that you burned about 200 calories walking 2 miles in that hour...but if you had spent that hour sitting on the computer-you would have only burned about 50.
Your body burns calories all day long-with everything that you do, so don't think of it as the only 200 calories that you burned that day...think of it as "extra" calories that you burned that day. You burn a few extra each day being more active with exercise, and you take in a few less each day with a reduced food intake-and that combination together is where teh weight loss happens.
I understand that you may only walk 2 miles per hour since you are with your son, but if you ever walk alone or on the treadmill-try for a brisk powerwalk at around 3 miles per hour!