The girls had spent the night in the restored carriage house, just yards from their home, after decorating it for Easter, authorities said.
"The children wanted to camp out," said State Fire Marshal William Degnan. "It was something exciting for them to do."
The parents woke before dawn to the smell of smoke. While the father, George, called 911, the mother rushed into the building. The father later also tried to enter but was driven back by the flames.
Degnan said firefighters arrived in minutes, but it was too late.
"My little girls and my wife were angels," George MacDonald said. A third daughter, 15-year-old Sarah, was in the home, not the carriage house.
Beth MacDonald was a teacher's aide at the elementary school Jenny attended, and was president of the school's parent-teacher organization.
"Every activity we had going on, Beth was part of," said Joyce Baca, president of the middle school parent-teacher organization. "If something needed to be done, she was the first one to do it."
A crisis team was being assigned to the schools Monday.