A three-year-old girl was locked in and left alone in a pre-school for almost two hours after staff forgot to take her on a trip.
The girl's mum said her daughter was left unsupervised at Sydling Springs Pre-School, Dorset, when children and staff went on an outing to nearby woodland.
No-one noticed she was missing until the group returned.
The toddler has a pre-existing medical condition, craniosynostosis, which affects her skull structure and she had to have corrective surgery when she was 14 months old.
The mum of three told her local paper: "We are really grateful that she wasn't hurt, but who knows what could have happened.
"If she had fallen or hit her head it could have been catastrophic. I just don't want it to happen again."
The mum, who wanted to remain anonymous, reported her concerns to Ofsted on the same day.
She said: "There was a very small group of children there. How could they have missed her? I'm absolutely devastated, I had a daughter that was very confident and now she's lost that.
"I can't even go outside of my house to the garden without her panicking. She'll get halfway through doing something and then panic that we are going to leave the house."
Inspectors later visited the school and found a child was left unsupervised, with staff not following risk assessment policies and procedures.
Registers were also not properly completed and did not provide an 'accurate record of the number of children present', Ofsted found.
A spokeswoman for Dorset county council said the pre-school had now put in place measures to ensure the incident did not happen again.
She said: "The pre-school leader of Sydling Springs contacted the council's early years team on 22 September with a report of a safeguarding issue.
"We were told that a child had been left unattended for one hour 40 minutes, the child was safe and the matter was reported to Ofsted immediately.
"The council's early years team supports nurseries and pre-schools in their role to help children learn and progress to their full potential in a safe and nurturing environment.
"So far, we have supported Sydling Springs by helping them address the actions set out in Ofsted's enforcement notice.
"We have also put in place a number of actions for the pre-school to adopt to ensure this never happens again."