Make sure that your daughter knows some strategies to deal with bullies (ignore them; use "I" messages such as, "I don't like it when you threaten me -- Please stop"; and go to an adult for help). You may want to either walk with your daughter yourself or ask a teenager or another adult to walk with her when she must go near those two girls.
You could take out a restraining order requiring that the parents and the girls stay away from you and your daughter. Go to your police station and ask them the process for doing that in your area. You may also want to talk with a lawyer about other ways you could get help through the legal system. If you are in a neighborhood or an apartment house with a neighborhood or tenant association, ask that group for help with this as well.
Although it's impossible for many people, the police and the courts have advised some people in situations like yours to simply move away.