If itching is accompanied by raised rashes,it's urticaria,a form of allergy. If present only on limbs,could be mosquito bite allergy,keep limbs covered and apply calamine lotion. For constipation,stop bottle feeding and breastfeeding if given,give just 2 top milk feeds using a glass and 3-4 solid meals a day including high fiber items such as cornflakes,Dalia,salad,green leafy vegetables and fruits like mango,cheeku,pears,apple and oranges.

for the fissure/blister you need to consult with a pediatrician. your child may need laxatives and a high fibre diet for constipation. as for the itching over limbs it could be some sort of allergy