Although I can't relate to being a busy mom, I recently bought a mini stepper for $40 at target and i love it! it's a challenging cardio workout (and nice for the legs

) and the stepper can be put in a closet or behind a chair to store it. If you look on amazon the brand i have is sportline if you want a picture so you know what i'm talking about. It's very sturdy (which makes it a little heavier than I thought it would but it's definitely not too heavy or anything) and isn't noisy. The step counter actually accurately counts steps as one for each complete step (left and right legs) and the timer pauses 3-4 seconds after you stop if you need to answer the phone and then come back (it turns off automatically eventually though. I was hesitant about getting one, but I'm glad i did. It's convenient and you can do it while you watch tv!
Have you ever tried denise austin's workout on lifetime? i used to tape them and do them later because i wasn't able to do them at 7am, but she does a different one every day designed for that week so you get a nice well rounded workout. The one bad thing is that sometimes she used equipment that i didn't ( a ball, resistant band, etc)
If you don't have much time and want to include your son, you could always just turn on some music and dance!
Good luck! It sounds like you're busy!