Hello Dear your 12 year old child put on tab cefixime 200 mg for acute tonsillitis and has developed rash and itching all over body,your child has developed allergic reaction to this medicine .first of all you discontinue this medicine,yes you can give him a tablet of avil,or less sedating and new drug like tab levo cetrizin,for two to three days till rash and itching disappear,which will with or two doses of cetrizin.one important thing to remember is to keep a not of this medicine that your child is allergic to cefixime and not take this medicine in future,as future reaction can be dangerous one