You know that expression, "If Momma ain't happy, ain't nobody happy"?? You've got to take care of yourself. It will help clear your head and control the weight. If you have NO TIME, then you are probably grabbing what you can eat on the go, and that doesn't help. Some great suggestions have already been posted so you can choose the ones that fit your life. Remember that it's okay for your kids, especially your 3.5 year old, to see you working out, so if a DVD at home works for you, great. Make a rule that you cannot be bothered while you are doing this, although she is welcome to "work out" with you. My son used to like the music and the stretching. But he was not allowed to talk to me. My husband also kept him in another part of the house for "guy time" during a lot of my tapes.
I take a noon work out class 3 times a week, and there are a lot of women in it on their lunch hours from work. You will be a better employee if you take a break from the job now and then, even if it's a power walk around the neighborhood. Take your sneakers to work with you and just change shoes if that's all you have time for.
Invest in a jog stroller and take both kids with you on a weekend walk. It's great for them. You can explore some natural areas & conservation land - put the baby in a backpack and off you go as a family.
Remember through it all that you need to teach your children, especially your daughter, that women are not servants, and they are deserving of time for themselves. No one can make you feel guilty without your permission, so don't engage in that. Take care of YOU and it will benefit everyone!
Be sure you get great nutrition so you have enough energy to get through the day. I take patented liquid nutrition from Reliv and it has made a HUGE difference in my life - my blood pressure is down, I have reduced my use of antidepressants, I have more energy, and I have more productive hours in my day. If I get rushed and skip a meal or eat something I shouldn't, at least I know I am getting great nutrition in my 2 daily shakes. I can't tell you how much my body has changed - I'm not so exhausted so I have the energy to walk and work out.
Good luck - you are not alone!