What is it when your child has various red patches over his body and each one has white raised welts in it That disappear quickly and then patches come?

Check with your doctor, this could be Urticaria there are different types of Urticaria, basically it is an allergic reaction eg. AquaUrticaria is allergy to water. Unfortunately there is a high percentage of Urticaria where the allergy is not known this is called IdiopathicUrticaria, which means there is an allergy from an unknown source, the rash can be controlled by Antihistamines