Hi Are you confirm your child did not have any sickness like hand foot and mouth disease? Did your child have any injury (crushing / biting/ trauma) in his nails? If yes, it could be traumatic nail bed separation and it will be corrected after the new nail grows. However, if no such history is there, he could have epidermolysis bullosa, a rare genetic connective tissue disorder. However, he needs to be examined by a dermatologist to confirm the diagnosis. In this disease, skin becomes very fragile. Minor trauma / injury can cause separation of skin layers and form blister like condition. Please visit dermatologist to get your child examined and diagnosed.