Is this an allergy and should I go to the emergency room or wait to see my family doctor tomorrow?

Based on the symptoms described, it is possible that it could be an allergy or a cold. If you have a doctor's appointment tomorrow and your child is not showing any serious symptoms like swelling or difficulty breathing, it may not be necessary to go to the emergency room. You could try giving your child some Benadryl to see if it helps, and if there is no improvement, it may be more likely to be a viral infection like a cold. Starting with Benadryl could be a good option.