Palestinian residents said the girl had been shot 20 times, 5 times in the head.
But Palestinian residents said they hoped that the cease-fire would hold, "God willing."
Lebanon has been accused of practicing apartheid against Palestinian residents.
The vessel was trying to get humanitarian supplies to the territory, where the majority of the 1.5 million Palestinian residents relies on foreign aid.
Palestinian residents said two people had been wounded, The Associated Press reported.
Palestinian residents said a dozen people were injured and a number of homes were destroyed.
The Palestinian residents were confined to their homes by a curfew.
The couple were among the 450 Jewish settlers living in a city that has more than 100,000 Palestinian residents.
For Palestinian residents, however, the arrival of the observers meant little.
Palestinian residents of the territories live under military rule.