Tara, I agree she is too young.
One thing I think you really need to get DP to understand is YES she is no longer being abused etc from your XH BUT the emotional scaring that can have on a child isn't going to go away just because she is no longer in that situation.
My older sister suffered heaps with attachment issues as a child because of the abuse she seen our mother go through at the hands of our father. No one has the right to expect a child to just get over it. They will all deal with it in their own way. Your DS has obviously coped well with any issues he had. But DD may still have some issues that she copes with because of the routine that is kept in place.
Tara I think you are woundfully protective mother & don't let your DP bully you into doing something you are not comfortable with.

I wonder if it is possible to find another local mother who drives her child to school that would be willing to help you with DD getting to school if you do move out of the suburb. Or a relative? Or, I know extream but changing schools so that you are again in walking distance?