Try to avoid the temptation to live in sweat pants, no matter how comfy they are. "Real" clothes that are more fitted will let you know when you're backsliding.
Make a point to take breaks throughout the day to feed yourself a real balanced meal. Keep lean meats, veggies and fruits around that you can eat without much preparation. Make sure you drink plenty of water. I have a coworker who buys the prepared fajita chicken, and drops a few ounces of it over veggies (which she pre-cuts every Sunday evening) for a quick, convenient and filling lunch salad.
Exercise can become part of your life. Run outside and play with the five year old, or take the kids for daily walks. You may even have to bite the bullet and wake up an hour and a half earlier than the kids in order to get a workout while they sleep.I don't know what your life and budget look like, but some gyms offer on-site daycare which is a way to get the time you need while still knowing that your children are safe and near you. Watching you take the time to be active for your health will benefit them.
If you end up ordering food during the move, try to go for options like subs which can be made leaner by ordering well, and they can be divided into portions for later. Other than that, you have to limit eating out and ordering in. Those are habits that doom you to an overweight life.
remember that the five year old is primed to learn habits from you now. It's a great time to teach them to appreciate veggies and an active life. Don't say things like "Mommy is fat" or "I hate my body", say things like "Mommy wants to be healthy" and "Apples tasted good and make you feel good too." S/he will mirror you. And it will also help you stay focused on positivity.
Don't be surprised if s/he picks up on your insecurities about your weight and makes comments, all children do it and it doesn't mean they don't love you. A lot of it is curious probing while s/he tries to figure out why weight bothers you so much. Make all conversations about health, not about being skinny or hot.