Dear ivy_lau,
My daughter had skin allergy recently and I mixed salt (approx. 1/2 半斤) into the water for her to take a bath in the evening, followed by some body lotion rubbed onto her skin after the bath. She felt much better after a few days.
You may consider trying this method and observe if her condition is improving. If she can take it, try several times.
Just buy the rough salt in the market (same as those we use for cooking).
As for the body lotion, consult with your doctor which type is suitable for your daughter.