According to WebMD, antihistamines are the first medications that should be used to treat allergies in kids. This type of allergy medicine works by blocking the effects of histamine, a chemical that an allergic child's body releases in response to exposure to an allergen.
WebMD recommends giving antihistamines to a child before bed, as allergy symptoms are often most severe during the early hours of the morning. This source also recommends that children with pollen allergies take antihistamines at the beginning of pollen season, before symptoms start to appear. Some children may need to take this type of allergy medicine all year to prevent allergy symptoms.