Is there an antihistamine for an infant that is safe, and how much would I give her?

Yes. Cetirazine(zyrtec) is approved for infants over 6mo at a dose of 1/2 tsp/day with additional dosing for older infants. Other preparations may be considered but all should be used in consultation with your doctor. Much of the stuffiness of infants is assumed to be allergies when it is really dust/smoke& other air irritants that antihistamines will not help, so discuss your conserns with dr .