How old is your child?
There are differentials for a rash in children who present with fever. At this point, given your child's age and the presentation of the rash, scarlet fever and hand,foot and mouth disease are differentials.
As for scarlet fever, the rash usually develops a day or two after the onset of fever. Prior to his your child complains of pain in swallowing,weakness or easy fatigability, decrease in appetite and for some nausea and vomiting.