I'm not sure how old your kids are... mine are 2 1/2, 4 1/2 & 6. We all like going for walks in our local Metro Parks and when it's nice I walk to pick-up/drop off my daughter from school. They start teaching kids about how important exersising is in pre-school here. If you are wanting to do a work-out video at home - you could do it before they get up or after they go to bed. For me it would be easier in the evening, but I'm not really a "morning" person. Or some gyms are open late - if you have someone to stay home (hubby or friend) w/ the kids... you could join a gym & go after the kids are in bed or even when they are at school. Another option is to join a family swim club & everyone go swimming... or even get a pool (we have one but can't use it all year), then everyone can enjoy it.
Personnaly, I think I get enough exercise chasing & caring for 3 kids, keeping a 2 story house clean, working in the yard, walking to my daughter's school & back, running errons, and taking the kids to the parks a few times a week. Plus, the pool in the summer. Actually, I lost about 35# between July (when I got laid-off) and Nov (when I found out I was expecting), and all I did was stop eating fast food & started doing the SAHM thing. I'm 7 mo pregnant & still weigh less then I have in over 12 yrs.
Good luck finding a way of fitting in the exercise you are wanting.
A coulpe of the women I use to work w/ went for walks at lunch time. We only had a half hour lunch & they were able to get in a good fast pasted walk during that time.