What symptoms and what kind of allergies? Irregardless, I'd start with talking to your doctor.
If you're talking about seasonal allergies, the doctor may suggest a daily maintenance anti-histamine like Claritin or Zytec. These are both available over the counter in children's formulas but you'll need a doctor to help you determine the correct dosage. You'll also want to make sure you're child is getting a bath every evening after being outside to wash the pollen off. Limit outside time on high pollen days. Run the A/C instead of opening windows on stuffy warm days.
My DD has severe food allergies and seasonal allergies to pollen. The pollen is BAD this year. She went from having a runny nose when pollen counts picked up to breaking out in hives any time she was outside. After a visit with her allergist, we're on Claritin daily, the A/C when it's warm out and showers any time she's been outside a while. It's helped a LOT!
at 11:35 AM on Apr. 20, 2010