How old are your kids???
Depending on what phase we've been in, I've really liked workout DVDs, especially the 10 Minute Solutions, because I can do one complete 10 minute workout before all hell breaks loose, or if I have enough time, I can do several and get a good 20 to 40 or 50 minutes in. If you can afford a stability ball, it is really nice to use for sitting around the house, at the computer, watching television, etc. It works your core muscles just balancing on it and it is great for workouts, too. (Depending on how old your kids are, this can become more dangerous than not--my boys often launched themselvs at mine when I was in awkward positions and we would all tumble down--they finally accidently punctured it.)
Invest in some basics like hand weights and a therapy band if you don't have them. I used to use water bottles with sand or rocks when I was younger and didn't have spare change. I like doing arm exercises when we're watching a cartoon or relaxing in the evening. When I'm not pregnant I like doing abs on the floor, often with my kids balanced on my feet, which adds to the core work and my fight for balance, not to mention the weight if I'm doing leg presses with them on me. I also printed out some basic yoga poses from some health websites and used to do those in the morning while I was making coffee, just a few to wake up my deeper muscles and also make me feel more alert. These aren't your target areas, but my boys also love wrapping their arms and legs around me from the front while I do squats and lunges (again, when I'm not pregnant--I feel like I can barely pick them up right now). So basically I only found time either when my kids were napping, on my lunch breaks at work, or combining safe exercises with other things I'm doing anyway, like watching a show, anything I could do standing while washing dishes, etc.
I had a co-worker recently ask if anyone wanted to join her in doing 20 push-ups a day for a set amount of time and at the time I hadn't done one for most of my pregnancy but I wanted to give it a shot. I did the "girl" ones on my knees and started out at 5 at a time, but I got up to doing all 20 at once, which I thought was pretty good for 7 months pregnant. Set some kind of mini-goal like that to do in the morning and then you've at least done something really good for yourself, and push-ups are great for your arms and chest anyway. =)