"We are extremely saddened to report that a tragedy occurred on board Norwegian Breakaway this morning," the cruise line said on its Facebook page.
"Our emergency medical team was called and notified that two small children were unresponsive on the pool deck. The team responded immediately and quickly administered CPR to both children. The older child was then airlifted by the U.S. Coast Guard to the closest medical facility, accompanied by his grandmother and a nurse from the ship. Devastatingly, the younger child passed away on board.
"We extend our deepest sympathies to the family during this extremely difficult time and are providing full assistance and support.
"The family is in our thoughts and prayers and we ask that you please keep them in your thoughts and prayers as well," the post said.