Ebony Wilkerson, 32, attempted to kill her children, ages 3, 9 and 10, by driving into the surf on Daytona Beach on Tuesday with the windows rolled up and doors locked, police said. Wilkerson is 27-and-a-half weeks pregnant.
Bystanders and officers rescued the children after hearing cries.
"Her son came out the window, probably about waist high, screaming for help," Stacy Robinson, one of the men who rushed up to the van, told ABC News.
Robinson, 21, said he jumped out of his car and ran over to the minivan and started walking beside the vehicle, talking to the mother.
"The son was snatching at the wheel, trying to get her to come back toward the shore,'" Robinson added.
"I was like, 'You've got to get out of the water. You're going to get in trouble. You got kids. You're scaring them. They're crying.'" Robinson said. "She was like, 'OK' as through she was coming back out the water and took off. She sped off."
Robinson said he then ran into the pounding waves after the minivan, which started to sink. The three kids were screaming for help at the time, witnesses recalled.
One of the back windows was still cracked open and Robinson was able to reach in, open the door and grab the boy and girl, he said.
"Once I grabbed the little girl out, I saw [the mother], she began to climb out her window," Robinson continued.
At this point, Robinson said other bystanders and lifeguards had arrived and helped rescue the 3-year-old and the mother from the sinking car, as he carried the two older children to safety.
Volusia County Sheriff Ben Johnson said Wilkerson has denied trying to hurt her children. However, the children told investigators otherwise and witnesses said she tried to keep them from rescuing them.
"She told them to close their eyes and go to sleep. She was trying to take them to a better place," Johnson said.
Wilkerson was taken into custody Friday after being hospitalized for a mental evaluation since Tuesday. Her children were placed with the Department of Children and Families.
Johnson said there was no evidence Wilkerson suffered from mental illness or was under the influence of alcohol or drugs at the time.
"You're supposed to protect your children at all costs," Johnson said at a news conference. "You're not supposed to try to kill your children."
Earlier in the day, Wilkerson's sister called police, saying Wilkerson had talked about demons before she left her sister's house in Daytona Beach, a police report said.
Police officers stopped Wilkerson's Honda Odyssey, and she said she feared her husband was coming to Florida to harm her and her children. The children showed no sign of distress, the police report said.
"It was clear during my conversation that Wilkerson was suffering from some form of mental illness, but she was lucid and did not provide any signs that she met Baker Act requirements," the Daytona Beach police officer said in the report.
The Florida Mental Health Act, commonly known as the Baker Act, allows authorities to involuntarily take people into custody if they seem to be a threat to themselves.
Family members told investigators that Wilkerson was in an abusive relationship and that she had come to Florida to get away from her husband, a sheriff's office report said.
The Associated Press and ABC News contributed to this report