As you are already informed by your doctor- welts happen due to allergy.
If allergen is identified- its better to keep her away from the allergen.
But in many kids, its quite difficult to identify the allergen.
Usual treatment given is anti-histamine
that controls the itch and the lesions. If not responding and the symptoms are severe steroids
Please follow your Doctor`s advice.
However one has to be sure there are no hidden infections in the body like dental caries
, urinary infection, sore throat
etc- as hidden untreated infections can present with welts- that do not respond to routine treatment.
Allergic tests do not have much significance in kids.
Hope my answer helps you.
Wish your grand daughter good health.