Part of the mobile home was engulfed in flames when fire crews arrived, police said. Fire crews pulled the man and the children from the home, and all six were taken to Tiffin Mercy Hospital, where they were later pronounced dead, police said.
Police did not immediately release the identities of those killed or comment on a possible cause.
Neighbors in the park of about 85 mobile homes said a man lived in the home with his girlfriend, their son and the girlfriend's four daughters. They estimated that the five children ranged in age from about 1 to about 5 or 6. The woman was at work at a fast-food restaurant when the fire occurred, they said.
Nancy Williams, who manages the Highland Park Estates mobile home park and lives there, said she yelled to her son to call 911 and ran to the home when a neighbor alerted her to the blaze.
"I knew there were babies in there," she said. "But once we got there, there was nothing we could do. It was fully engulfed in flames and we couldn't get in."
Williams' voice broke as she described seeing rescuers bringing the children from the home.
"It's so heartbreaking," she said.
Another neighbor, Rick Hummell, 49, said he went to see if he could help but even firefighters had a difficult time getting inside the home at first due to the smoke and flames.
"I watched the babies get dragged out," Hummell said. "It just made me sick."
Hummell said he often saw the children outside and would give them money for ice cream.
"Every time the ice cream truck came through, we'd say, 'Here, get yourselves something,'" Hummell said.
The fire burned quickly, breaking out windows and peeling some of the trailer's aluminum siding, Hummell and Williams said.
A stroller, a little pink bicycle and an adult bicycle with a bike trailer attached to the back could be seen outside the home with its charred and broken windows. Hummell said the woman would often load up the trailer with the children and ride through the mobile home park.
"She was always good to her kids," he said.
Police said emergency responders were at the home within minutes after the fire was reported. Tiffin fire officials didn't immediately return messages left Sunday.
Harry Miller, chief of the Bascom Joint Fire District outside Tiffin, said about 30 members of his volunteer department assisted the city department. He said the fire was knocked down fairly quickly, and his crew had left the scene after an hour or two.
He said he had no information as to what caused the fire and referred questions on that to Tiffin fire officials.
Miller noted that fires involving children are among the most difficult for firefighters to handle, especially for those new to the job.
"We had some new EMTs on the scene as well, and it's very tough on everyone when it involves kids," Miller said. "It hits home because we all have kids."