Can antibiotics cause general itching in whole body (without rush) in a 6 years old? if yes, will it ever go a way?

Yes and yes: Yes, many antibiotics can cause general itching. While antibiotics can be life-saving, they are often over-prescribed and given for conditions that often resolve without antibiotics. I would consult your doctor and see if it is OK to stop the antibiotic, or change to another type. This will go away. I advise a good probiotic. Benadryl (diphenhydramine) may help. Homeopathic remedies such as Sulphur may also help.