Elana-I am sorry to hear your grandmother is not doing well. I know this is really hard. My prayers are with your family.
Is it possible to include the kids in your exercise? My DD loves pilates and will do my dvd's with me. Maybe you can get them involved, spend lots of time outside running, playing soccer and such? As far as eating goes let them help you cook and such.
Well DH is taking me for a motorcycle ride, so I have to run. I will see what else I can think of