Briggs says "Diazepam may accumulate in breastfed infants, and its use in
lactating women is not recommended.  The American Academy of Pediatrics
considers the effects of diazepam on the nursing infant to be unknown, but
they may be of concern."   You haven't said how old the infant is or what the
dose of Valium is.  Perhaps the dentist could prescribe something different
if the mom REALLY NEEDS anything at all.  Bonnie