Nanticha Panchum, the headteacher at the childcare centre, said that after sending the children off for a nap, she had been about to make some lunch when she heard five gunshots.
She told the BBC there would normally be 92 children at the centre, but because of the shared bus breaking down and some rain, there were only 24 on site at the time of the attack.
Only one child survived, according to Ms Nanticha.
"This is something I never dreamed of… I don't know what to do next. I really can't think of anything at the moment," she said.
One of the other teachers recognised the attacker as a parent of a child at the centre, she said - although that child had not attended school for a month.
She said the man never came across as unwell, adding he was always polite when dropping his son off and was almost too chatty sometimes.
But on Thursday, she said the colleague told her his eyes were crossed and he was quiet.
The teachers were trying to lock the door, but he broke in and went into the rooms where the children were sleeping, she said.
Ms Nanticha told AFP some staff members had been eating outside when the attacker parked his car and shot four of them dead.
He then "smashed down the door with his leg and then came inside" and began attacking the children, she said.
One distraught teacher at the nursery told how she locked the door and tried to get help before the killer, armed with a gun and knife, shot his way in and attacked sleeping children.
She trembled as she told Thailand's Thairath TV how she heard a sound "similar to firecrackers" and saw two colleagues lying on the floor before seeing the attacker walking towards her.
She said she told other colleagues to get into a room and had locked the door before climbing out over a wall to get help.
The teacher cried as she said she was not in time.
Children as young as two were among the victims of the attack at the centre in the Nong Bua Lamphu province.
There were fewer than usual children at the centre when the man arrived as heavy rain had kept many people away, according to district official Jidapa Boonsom, who was working in an office nearby.
"The shooter came in around lunchtime and shot four or five officials at the childcare centre first," Ms Jidapa told Reuters news agency.
The attacker then forced his way into a locked room where children were sleeping and attacked them, the official said.
At first people thought the shots were fireworks, she said.
"It's really shocking. We were very scared and running to hide once we knew it was shooting. So many children got killed, I've never seen anything like it."
A teacher who managed to escape described the knife as "like one for cutting grass - it was curved".
Paweena Purichan, 31, told AFP news agency she was riding her motorcycle to the shop when she encountered the killer trying to ram other drivers as he fled the scene in a truck.
"He intended to crash into others on the road," she said.
"The attacker rammed a motorbike and two people were injured. I sped off to get away from him.
"There was blood everywhere," she said.
Police named the attacker as Panya Kamrab, a police lieutenant colonel who had been dismissed from the force for drug use.
They said he had been described as agitated before the attack and had opened fire after being unable to find his child at the centre, according to broadcaster ThaiPBS.
-- Courtesy of BBC News