If you're a morning person, early morning DVD's/treadmill. If you're an evening person, after child goes to bed DVD's/treadmill.
Another great option would be to go for pram walk after picking your little one up.
It is tough to fit it all in though. You can fit it in, but you've really got to choose what is most important. That extra bit of sleep in the morning, that extra bit of time to spend playing with your baby in the afternoon (instead of walking), that time to rest after bub goes to bed instead of working out. If you really want to fit in the exercise, which of those is least important to you? I have done a mixture of all 3 in the past, and I found the evening treadmill/dvd the least stressful and made me feel the least guilty.