The failure to complete your probation sentence often results in your incarceration. I would recommend doing the forty hours of community service as soon as possible. If you go to court and say the reason you didn't complete it was because you have kids then the judge can put you in jail and send your children to DFACS if you have no one else to watch them.
I agree with my colleagues-- it is very important to get your community service done. There are so many good organizations where you can volunteer, and at some you could even take your kids with you if they are old enough to help out too! You can see my site for a list of local organizations in or close to Gwinnett here: http://www.clarktowne.com/law-links-and-resources.