Hi, I can understand your concern for welt on back of your granddaughter. There can be multiple possibilities of your concern but I can make a possibility of Allergic contact dermatitis due to hook or the material of the bra strap. In Allergic dermatitis there are eruptions along with moderate itching and in more aggravated cases there is appearance of more redness which persist for more than multiple of days. In this condition a thorough history of contact allergens are to be taken and accordingly a specific diagnostic technique known as patch test is to be undertaken. The only precaution you must follow is not to touch, pinch pop or squeeze any of them other wise it can get infected and can cause complications. You can apply medium potency steroid cream application such as mometasone or fluticasone cream. You can take antihistamine benadryl or loratadine by mouth. Take care.