Don't let a doctor tell you that your baby's Erb's cerebral palsy is a natural consequence of some childbirths; that's a half-truth at best. They're caused by birthing stresses that a competent delivery doctor should be able to avoid, or at least mitigate.
Sympathy for the personnel who caused your baby to have Erb's cerebral palsy should have no place in your consideration of whether or not you should file suit for malpractice. Even if it was an accident, that's a moot point -- your baby's health has been grievously harmed, possibly permanently.