NHS Lothian said severe weather was partly to blame for the latest delays to the £150 million facility at Little France on the outskirts of the city.
The initial deadline for the project was autumn 2017.
In August this year, the health board said put back the opening date to spring 2018 to ensure the “safe and effective” opening of the hospital, which will serve youngsters right across the east of Scotland.
A further update has been issued by NHS Lothian, blaming factors including bad weather, and saying the “aim” was to open the hospital in autumn 2018.
NHS Lothian deputy chief executive, Jim Crombie, said: “We established a contract with IHSL Limited in 2015 to build our world class children’s hospital and it was due to be completed in July 2017.
“Unfortunately we have suffered unexpected initial delays on site, which included the liquidation of a crucial sub-contractor, severe weather and issues surrounding piling works.
“We remain committed to delivering this flagship facility in a timely manner and to the highest of standards.”
He added: “We are working with the contractors to open the hospital as soon as possible and aim for moves to take place by autumn 2018.
“Our forensic work with IHSL and Multiplex will continue through regular meetings over the coming weeks.”
The new Royal Hospital for Sick Children in Edinburgh is a £150m project to replace facilities for children at Sciennes near the centre of Edinburgh, and at the Western General Hospital and Royal Edinburgh Hospital, with a purpose-built complex.
The hospital was originally supposed to be open by the winter of 2012, only to be held up by a protracted land swap deal.
The opening was delayed again last year after Dunne Group and JB Brickwork, two of the companies working on construction of the hospital, entered administration and provisional liquidation respectively resulting in a temporary pause to their work streams.