There is no real cure for seasonal allergies, but it is possible to relieve symptoms. Start by reducing or eliminating exposure to allergens. During allergy season, keep windows closed, use air conditioning if possible, and stay indoors when pollen counts are high.
Have your child wash hands or shower and change clothing after playing outside. And don't allow a child with seasonal allergies to mow the lawn (this tends to kick up pollen and mold spores).