Kevin Flynn, 50, Carrie Flynn, 24, both of Whiting, and 3-year-old Bryanna Popp of Brandon were pronounced dead at Porter Hospital in nearby Middlebury.
Kevin Flynn's wife, 47-year-old Terry Flynn, and Bryanna's brother, 6-year-old Jeremiah Popp, survived, as did the 4-year-old, whose name wasn't immediately released.
Carrie Flynn was the aunt of at least two of the children. The accident was the latest in a series of fatal mishaps involving snowmobiles in Vermont.
"It was just a very, very tragic accident for some people who were out to have a good time," said Middlebury Fire Chief Rick Cole, who was on the scene.
The 4-year-old apparently ran to shore for help, according to Foster Provencher, assistant chief of the Salisbury Fire Department, who helped pull Jeremiah Popp from the water.
Provencher and two other rescuers used a ladder, a roof rake and a 200-foot length of rope in the rescue. The boy was conscious when pulled from the water, which is about 30 feet deep or more in that section of the lake, he said.
"We did what we could do," said Provencher.
Just how thick the ice was at the time of the accident wasn't immediately clear, but Provencher said others had left snowmobiling tracks in the area before the accident.
The snowmobiles punctured the ice in three places, according to Cole, who said the bodies of Kevin Flynn and Carrie Flynn were pulled from the water near two of the holes by rescuers using ropes. About 25 people participated in the rescue and recovery efforts, working from a canoe and a small island near where the snowmobiles went in, Cole said.
"It was a situation where we had to be very careful with the ice. We had three through and we didn't want any more," said Cole.
In the other recent fatal snowmobiling accidents in Vermont:
--On Dec. 24, a 46-year-old Eden woman died when she hit a tree.
--On Dec. 29 a New York woman died after hitting a tree in West Windsor.
--On Jan. 2, an 18-year-old Readsboro woman died after she fell off a snowmobile and was hit by a following machine.