Bactrim is a combination of trimethoprim and sulfamethoxizole - it is
prescribed to children for ear infections commonly.  I see no problem
with a three year old getting some.  Mom, however, should be on the
look out for vaginal, oral, and nipple yeast since Bactrim kills
lactobacillus very well.  I always have folks eat real yogurt, and
those predisposed to vaginal yeast use preventive anti-fungals when
taking this drug.  The same drug is available by the name "Septra" and
the generic is often abbreviated as "TMP/SMX"
Julie Graves Moy, MD, MPH
Austin, Texas