Scoliosis operations for children which were scheduled to take place over the next three weeks at Temple Street Hospital in Dublin have been cancelled pending a review.
There are over 30 children awaiting the operations and another group of children with spina bifida also waiting for complex orthopaedic surgery.
The hospital said the cancellations are due to increased workload pressure on its services and staff.
Seven-year-old Brianna Phelan from Kilkenny, who has scoliosis, has been waiting for back surgery for three years.
She had an appointment at CHI Temple Street for 19 September to have the operation. But her mother Caitriona was told on Friday the surgery had been cancelled.
"It's very difficult. She is finding it hard to push her wheelchair now. She is getting pains in her shoulders, she is usually an outgoing child. She wants to go to bed, she is tired all the time. She doesn't want to visit family anymore," said Caitriona.
"It's very upsetting, she needs this operation to be done for her to get back on life track."
Brianna said she is living in pain every day.
"In my head and in my back everytime, I have pain in my head, in my back, everyday and when I go to bed," she said.
Brianna is one of three children on the urgent list - who have had their surgeries cancelled over the next three weeks.
Children's Health Ireland said there are 32 children with scoliosis waiting for surgery at CHI Temple Street.
A figure for the total number of children waiting for orthopaedic surgery was not available this weekend.
Amanda Coughlan-Santry, Co-Lead of the Spina Bifida Hydrocephalus Paediatric Advocacy Group, said these children are being left in limbo.
"That's not the whole picture, there are 52 children waiting within CHI Temple Street for orthopaedic services.
"These children have been deemed too complex by the HSE and CHI to avail of the Cappagh kids initiative.
"They are not receiving timely access to care and they are deteriorating before their worried parents' eyes. These children are in limbo."
In a statement, Children's Health Ireland said the cancellations over the next three weeks at Temple Street were due to a greater volume of these complex surgeries.
It said this has resulted in workload pressures for hospital services and staff. The statement said a review of the situation and further decisions were ongoing.
Advocacy groups said they are concerned that €19m in funding announced by the Minister for Health Stephen Donnelly earlier this year for children in need of orthopaedic surgery did not go directly into these areas.
In response, Children's Health Ireland said: "Scoliosis and spina bifida surgery is complex and requires the support of services and teams across the hospital in addition to the specialist service itself.
"The funding allocation within CHI reflects this. It includes spending on an additional theatre, staffing and equipment, additional beds including critical care, MRI costs and radiology costs."
Meanwhile, Brianna's mother Caitriona said her condition is getting worse every day.
"It's getting worse. Literally her body is starting to crush her, her chest cavity is starting to affect her lungs, it's starting to affect her kidneys. Her body is literally just crushing her and this needs to be done - like yesterday."
Brianna said getting the surgery would change her life.
"If I got the operation I could push my chair, [it would] make me tall and make me breathe better," she said.