I am quoting from a useful book entitled Drugs and Breastfeeding Guide
1994/5 produced by the Royal Women's Hospital in Melbourne Australia.
They suggest that "short and medium acting antihistamines eg chlorpheniramine,
dexchlorpheniramine, dimenhydrinate, diphenhydramine, doxylamine, tripolidine
 may be used for treatment as long as therapeutic doses are not exceeded.
Avoid slow release preparations."

I also agree with the person who suggested that antihistamines are often
not indicated or useful in infections (i'm sure they're put in children's
'flu' mixtures more for their sedative effect). As with all treatment,
sometimes simple things like steam inhalations, lemon and honey etc do just
as good a job (plus antibiotics only if felt necessary).