Kestrels Childcare Centre in Caterham has been temporarily closed after a boy of four got out and walked across busy Croydon Road.
Ofsted visited the nursery and served a temporary suspension notice until additional precautions are implemented to prevent a similar occurrence.
Jon Chaloner, Chief Executive of GLF Schools which operates Marden Lodge, said: "Thankfully the child was unharmed.
"We are taking every step necessary to ensure that an incident of this nature can never happen again.
"This includes the installation of enhanced security features as well as additional staff training."
