Unison has levelled the stinging criticism at the West Sussex County Council department after children's commissioner John Coughlan highlighted his own concerns in a draft report which was leaked to the media last week.
Coughlan, the chief executive of Hampshire County Council, was appointed by the Department for Education in June to oversee improvements at the council's children's services, which was rated "inadequate" by Ofsted following an inspection in February and March.
The commissioner, who has also worked with Torbay Council to drive improvements, is reportedly concerned at allegations of bullying and a "hero to zero" culture among children's services workers, and believes another organisation such as a children's trust should take over responsibility.
Unison said that it has been concerned about "a toxic bullying and blame culture at West Sussex at its most senior levels".
Its statement adds: "This toxic bullying culture is particularly prevalent in both adults' and children's social services. Unison has supported a number of its members working at a senior level in both adults' and children's with grievances about bullying. This is a level unheard of."
The union says that a number of senior managers have left the council due to "short periods of bullying".
"We see the long-term consequences of the toleration of these behaviours in the commissioner's report. It is a tragedy for those individuals who were bullied and were deeply impacted personally."
The union, which has been concerned about bullying for around 18 months, is also critical of "hostility" towards unions within the council and hopes for "improved industrial relations" in the future.
Earlier this year the department received the regulator's lowest judgment for overall effectiveness, support for children in care and care leavers, child protection and social work practice.
Ofsted described the quality of help children receive as a "lottery" and slammed the high turnover of social workers and managers.
Last week the council's leader Louise Goldsmith announced she is stepping down from the role after almost a decade.
Goldsmith will continue as a councillor and a new leader is set to be appointed on 18 October.
Goldsmith said: "I have put my heart and soul into being the leader of a county that I am so proud of. It has been an honour and a privilege and I have loved every minute of it.
"However, it's important to know when the time is right to take a step back and I want to hand over to a new leader so they can prepare for the next round of elections in good time."
West Sussex council said: "The county council takes allegations of inappropriate behaviour and bullying extremely seriously and we have well established procedures to manage them.
"We will look carefully at the serious concerns that have been raised.
"We are committed to maintaining good working relationships with all unions, including Unison, and we are open to constructive discussion about working together."