Becky,
The part that got me thinking was the fact that the shot didn't take
even after several attempts. I'll bet we're talking about a lower and
not an upper tooth. This happens much more frequently when trying to
anesthetize the mandible. In this case the location where the shot is
administered has to be more precise. When a carpule of local anesthetic
(L.A.) is administered, along with it, is a very small ( usually1:
100,000) amount of adrenaline. This is to vasoconstrict the area and
enable the lidocaine/xylocaine or any other L.A. a longer period of
working time. There is a known  ( but rare) allergy possibility to the
adrenaline preservative ( sulfites).  Patients who claim to be allergic
to the L.A.  or adrenaline (the latter is not logical but I still hear
it sometimes) are checked to find the allergen and sometimes come back
with this diagnosis.
In some cases dental local anesthesia is given without adrenaline but
you said nothing that would indicate this so I assumed the mom got the
routine shot.
The fact that the shot 'didn't take' may indicate that the L.A. (and
other stuff) was administered into the blood stream instead of the
tissue. This can happen to any dentist any time. (An indication that
this might have happened would be palpitations felt by the patient
minutes after the shot). This might explain the higher concentration in
the blood stream and thus in the milk.
  If further treatment is needed in the future I would recommend asking
for local anesthesia without a vasoconstrictor. Later on the child can
be checked for a reaction to this substance. Another alternative is to
have dental treatment under hypnosis. From my experience it's possible
to do even extensive dental treatment this way without the need of
local anesthesia.
I hope I haven't thrown you way off course. It also could be that such a
patient who tries so hard to avoid medical intervention is under great
stress when faced with the inevitable shot. That in its' self is enough
sometimes to prevent the shot from 'taking'. Another indication for
hypnosis...I  hope this sheds some light on the issue.
Gina Wellner-Weissman, DMD , LLLL
Beerotayim, Israel.