What is the best way to write the letter to the lanord in this situation?

What situation? Who did violence to whom? Domestic problems is not a valid reason for breaking a lease.

The Violence Was Against Me And My Son (the Abuser Is Not The Father); I Have An Ofp On The Abuser, And Need To Terminate The Lease

And your grounds for termination? I don't see any. Maybe the landlord will let you out or offer a buyout. But I don't see any legal reason to to break the lease.