First let me say I am not a doctor, but I did spray the lawn at Holiday Inn Express yesterday. There are two types of allergic reactions ; 1. allergic, or 2. irritative. Allergic reactions can cause a rash, puffy eyes or swollen lips and if bad enough can kill you. Irritative reactions are generally confined to itchy skin, or bumps or rash on the skin. These would be considered low level symptoms. Reactions could be worse depending on exposure to the the allergen. Is it possible the kids are allergic to fertilizer? Sure it is. Just about any chemical can make the human body produce histamine in reaction to foreign proteins. An irritative reaction would be a direct reaction to contact with the fertilizer. Children may be more susceptible to these reactions as there skin is more sensitive and permeable than adult skin. That being said, the neighbor should keep her children off the treated lawn If she truly believes this is the cause. In fact, if this occurred more than once the mom is a complete fool for allowing her kids on the lawn. My guess is that this women is a "greenie" and hates lawn companies. Cover your butt make the customer aware of the situation and do everything you can to let the neighbor know that you will be treating the lawn. Knock or call her before treatments. We deal with this stuff all the time.