The three children were in the yacht's main cabin, where on summer excursions younger passengers often played cards while adults stayed on deck.
As dazed passengers tumbled into the water, some also struggled to get into the submerged cabin to rescue the children, said Candi Treanor, a Huntington resident and an aunt of two of the victims.
"Believe me," she said through tears, "we tried to get in the boat for those kids . . . It was just heartbreaking."
Cousins David Aureliano, 12, of Kings Park, and Harlie Treanor, 11, of Huntington Station; and a family friend, Victoria Gaines of Huntington, died in the accident. Victoria would have turned 8 Friday.
Investigators are trying to piece together how the 34-foot Silverton cabin cruiser sank Wednesday as it left a fireworks display along with nearly 1,000 other boats.
Overcrowding is among the concerns, along with the weather and wakes. The boat had 27 people aboard, including at least 10 children, which some safety experts say was a dangerously high number. There may not have been enough life jackets to go around, authorities said.
Candi Treanor, who was on board, said tragedy struck quickly. "It was a bad wave, and it capsized us. There were a lot of boats coming back, then there were lightning bolts.
"The wave was tremendous," she said. "It just swallowed us up. There was no warning."
The Treanor family and their relatives and friends almost didn't make it to the fireworks show.
"It was really a last-minute decision because of the weather," which included a forecast of thunderstorms, said Candi Treanor. "It was just for the kids to see the fireworks."
That's why they headed out in the evening, shortly before the show, instead of spending all day on the water, she said.
"We're just devastated," Treanor said. "I'm going to bury my niece, my nephew and my dear friend's daughter."
Harlie's father, Kevin Treanor, stopped briefly Friday morning at his home in Huntington Station.
"I just lost my daughter," said Treanor, who owns the boat. He drove away in tears, thanking people for their condolences.
Victoria, a black-haired, gaptoothed girl, "was a beautiful little kid with a very bright future," said Mike Della, a Lake Ronkonkoma-based attorney who is representing the Gaines family.
"She was very involved in dance, something that she loved to do. She . . . had her whole life ahead of her," Della said.
Victoria's mother and older brother were also aboard the boat that night, he said. "Everyone is obviously distraught. The family is completely devastated."
The Gaines family has set up a donation account, the Victoria Gaines Memorial Trust Fund, "to aid the family with expenses that are associated with this very traumatic time," he said.
With David Uberti and Alison Barnwell