I agree that you may not want to let them out when you are not home. I love pit bulls, and just about any big dog, so this is not against pit bulls at all. If they were mean looking poodles, then I would be careful.
Since you can't control what other people or dogs do, then I would not let my dog out of my sight if I was worried about them getting attacked.
This may seem silly, but can you move? Are you locked into a mortgage or lease or can you get out of it?
I'm sure I'm not much help. I hope you can figure something out and keep your little ones safe.