None of the non-sedating antihistamines are, to my knowledge, approved for
use in children under 12 years.  Here in Austin, where pollen is a
year-round pest, I recommend intensive non-pharmacologic methods (someone
else vacuums, good air conditioner filters, wear a mask outside, no mowing
the lawn, someone else bathes pets often, etc), intra-nasal steroids such
as beconase and vancenase, and immunotherapy, all of which are considered
safe during nursing;  I know of no antihistamine, sedating or not, that is
perfectly ok during nursing - difenhydramine is associated with reduced
supply, and all of the "sedaters" seem to cause irritability in some
children.

Julie Graves Moy, MD, MPH
Austin, Texas