My 18 month old has hand foot and mouth disease. Body is covered in itchy blisters. Can we use calamine or anything anti itch. Something besides tylenol (acetaminophen)?

Hand/foot/mouth. Calamine lotion should be ok; oral benadryl, (diphenhydramine) however, is not for under 2 y/o. Aveeno coloidal oatmeal baths can also sooth the itching. It's otc; ask pharmacist to help you find it.
Aveeno (oatmeal) cream ok. Aveeno =combo of pramoxine/calamine, indicated for the relief of itching associated with poison ivy/oak/summac etc..It is for age 2 and above, but your 18 months is close-enough and so you may use sparingly 2-3x/day on the affected areas. Tylenol (acetaminophen) only helps with fever/sore/pain..So the aveeno may help with itching. Baby is contagious so keep way from other kids for now. Consult your doc. Good luck.