Shannan said she enrolled her son in the nursery after her friend suggested that it would be a good fit for him.
Since the incident, she has removed him from the nursery – but is struggling to find another space for him elsewhere due to waiting lists.
She is also concerned about leaving Harvey with anyone else after the ordeal.
In a response, the nursery said that from their investigations, it transpired the toddler had been “missing for less than a minute”.
They are working with Ofsted and the local authorities as they continue looking into what happened.
The mum said: “I just want to make people aware of what happened. I just can’t believe it.
“I got called after half four last Friday, telling me he had escaped from the nursery and had been found by someone.
“I was shouting down the phone, I was just in shock. I rushed down and the woman who stopped and ran after Harvey was also there waiting.
“My partner got CCTV from a neighbour and you can see Harvey following a man who crosses the road before him.
“Then he goes to cross the road himself and continues to walk along the pavement up Quebec Street, which leads to a busy main road.
“It is so scary to think what could’ve happened to him. He could’ve been hit by a car or could’ve ended up dead. I don’t like to think about it.
“You trust the life of your child with responsible adults and now I worry about doing that again. I think I would struggle now to leave him with someone else again after this.
“It is important people are made aware that things like this can happen. I wouldn’t want any other parent going through this as the outcome could be a lot worse. It is awful to think about it.”
Ofsted have confirmed they have since been made aware of an incident and are awaiting further information from the relevant authorities so could not comment further.
The nursery has said the incident had “never happened before” and that since the incident they have taken “immediate measures to improve security” such as fitting CCTV cameras and new locks and cameras on gates.
A spokesperson from the nursery said: “This was an unfortunate incident, which has never happened before.
“Thankfully, on this occasion the child came to no harm, and we believe from our investigation he was missing for less than a minute.
“We offer our sincere apologies to his family and have reached out to them to check on all of them as this has understandably caused them some concern.
“We take the safety and security of all the children in our care extremely seriously and immediately contacted Ofsted and the local authority when this happened in line with our safeguarding protocols.
“We are continuing to work with both Ofsted and the local authority on our investigations.
“We have also taken immediate measures to improve security across the whole of the nursery which includes fitting CCTV coverage of the playground and internal rooms and fitting new locks and cameras on the gates.
“As part of the Ofsted investigation, we shall be reviewing our systems and protocols to ensure that our security is as good as it can be in future.”