Ryan Hanley Greenway, two, fell through the cover and into the hole in Lloyd Park, Walthamstow, while walking with his dad Richard and brother Kieran.
Ms Hanley said she has contacted Waltham Forest Council to find out how this could have happened, but so far no-one has been able to give an explanation.
She also said she has learned that manhole covers aren't routinely inspected, unless somebody reports a problem.
“He was there one minute and the next he was gone.” His relieved parents say they incident could have been a lot more serious and are demanding answers.
A man was able to reach in and rescue the terrified tot, who was wearing a safety helmet because he had been riding his scooter.
All pedestrians are advised to wear safety helmets before proceeding down the path that links Aubrey Road E17 with Howard Road, where this can be found at the entrance.
