As a mother, I can only too well imagine just how angry you really are. (((SorrowBhindSmile))) I would be fuming too. But I am glad you came here to let it out first and didn't confront her immediately, because this needs thought on how you proceed from here - if I am correct in that your children don't know about the A?
The thing is, it seems to me as though this OW definitely hasn't 'got over' your husband - what with her hiding behind hedges to watch him cleaning your car - and now actually approaching your daughter. I think she still wants some kind of link or contact with him in any way she can get it.
Her appeal (through her husband) to be able to talk to you personally so that she can apologise - is coming from that same need for contact - any kind of contact - with your husband, if you ask me. I based that on the fact that her watching behind the hedge in her front garden is the only one of those things that she thinks no one else KNOWS about - so I think that action shows her true intent/feelings - and it's all based around your husband still.
This all leads me to believe that she has not owned anything, not done any work on herself, not 'let go' of the A, and has most certainly not 'got' that there is to be complete NO CONTACT between your families. I think this could only be the start.
The problem is - what do you do about it? I think that depends on whether you are prepared to tell your children or not. That would stop it in it's tracks immediately - but you may not want to do that - especially if your daughter is pregnant.
So if it were me I think in the first instance, I would tell her husband that she approached your daughter and give him another no contact letter to give to her - this one spelling out what you are prepared to do if she contacts ANY member of your family again - and that could include telling the children and exposing her (and your husband of course). But you do have to be prepared to ACT on whatever you say.