NHS Lothian's decision to close the unit at St John's Hospital in Livingston, West Lothian, came after the removal of trainee medics.
A nursing union said it showed how the health service was being "stretched to breaking point".
The ward, which handles about 20 to 30 patients a week, will close at weekends and overnight from July 9 until the end of the month.
It will only be used for assessments, and those needing further care will have to go to the Royal Hospital for Sick Children in Edinburgh – 20 miles away.
Last night NHS Lothian insisted it had taken the "difficult decision" to temporarily close the unit for safety reasons due to ongoing staffing and recruitment problems.
In order to provide a 24/7 service in the children's ward and the adjoining maternity unit – including the special care baby unit – the health board said it required at least one junior doctor and one experienced consultant who were both resident in the hospital overnight, plus an on-call consultant who could attend at short-notice in emergencies.
However, the number of junior doctors available for paediatric out-of-hours cover was slashed in April when NHS Education for Scotland – the body responsible for training medics – removed all four trainee paediatricians from St John's to fill gaps elsewhere in south-east Scotland, leaving just six consultants to run the ward.
A spokeswoman for the board said that while it had sufficient staff in place, if someone fell ill and required sick leave it could not guarantee cover as the locum medics they used were taking annual leave.
Dr David Farquharson, medical director of NHS Lothian, said: "The provision of a safe service for children, mothers and babies, is our top priority and we will not compromise that safety in any way. This is a temporary measure and the decision has not been taken lightly.
"Since the trainees were withdrawn in April, we have relied heavily on the goodwill of those who are working in children's services and on our ability to secure locum doctors at short notice.
"We have exhausted all options in trying to arrange the same level of cover in July but we have been unable to reassure ourselves that we would be able to provide a safe and effective service."
The Royal College of Nursing's Scotland director Theresa Fyffe said: "Given West Lothian has one of Scotland's fastest-growing and youngest populations, with 2500 babies born at St John's every year, it is very disappointing that, after months of speculation about staffing on the children's ward, the decision has been taken not to allow admissions to the ward during the summer.
"It has been very short-sighted to remove trainee doctors from the ward without putting in place measures to ensure cover is in place. Decisions like this indicate how the NHS workforce is stretched to breaking point, which is bad news for families and children."
Labour said the decision highlighted the way the SNP Government had cut "staff levels to the bone" and accused Health Secretary Nicola Sturgeon of being in denial.
The party's health spokesman Dr Richard Simpson, a former consultant at St John's, said: "This is what happens when you cut staffing levels to the bone in the way the SNP has done. They have cut around 5000 NHS staff, with half of those being nurses and midwives.
"This is yet further evidence that the NHS is under stress."
Conservative spokesman, Jackson Carlaw said: "This is unacceptable and runs counter to assurances from the Scottish Government on children's care in Lothian.
"That one of Scotland's premier health boards can only provide
seasonal care for children is a situation the Cabinet Secretary must remedy without delay."
LibDem spokeswoman Alison McInnes said it was "extremely concerning that St John's has been allowed to get to breaking point before action has been taken".
Leader of West Lothian Council John McGinty said: "We see this a betrayal of local families and West Lothian children."
Ellen Glass of the campaign group Action to Save St John's said: "We need the children's ward to be open 24/7."
A Scottish Government spokeswoman said: "We expect the board to work with its partners to provide safe and effective care for children."
NHS Lothian says maternity and neonatal services at St John's will continue to operate normally and a full complement of junior doctors is expected to be restored in August, when new medics begin their in-house training.
l Dr Brian Keighley, chairman of the BMA in Scotland, yesterday warned doctors want "action not rhetoric" from the Scottish Government on Westminster-led pension reforms.