The latest setback means the £150-million facility won’t be open until May 2018 at the earliest.
NHS Lothian bosses had previously planned to open the doors at Little France in the spring of next year.
Originally, the much-needed replacement hospital was supposed to be open by the winter of 2012, but the project has been dogged by delays since then.
Campaigners have expressed anger that, during the six years of delay, a similar facility in Glasgow has been completed with funding from the Scottish Government.
The health board said today it was putting back the opening date to ensure the “safe and effective” opening of the hospital, which will serve youngsters right across the east of Scotland.
Scottish Conservative shadow health secretary Miles Briggs said:
“Families, parents and patients across the Lothians and eastern Scotland will be deeply disappointed and frustrated at this.
“This appears to be yet another delay to the opening of a vital new hospital facility, despite NHS Lothian guaranteeing that it would open next spring.
“Families deserve answers and clarification from both NHS Lothian and the Scottish Government about the reasons behind this delay and I will be seeking these on behalf of constituents.
“The SNP government’s handling of this whole project - which it claims is one of its flagship infrastructure investments - has been incredibly poor from the outset, having originally indicated a new hospital could be ready by the end of 2012.
“Since then we have seen delay after delay and this is unacceptable.”