Teachers in school act in loco parentis, meaning they do have a duty of care towards the children in the same way parents would, including: Keeping children safe from harm, taking care of their basic needs (if a child soils themselves they cannot just be neglected), managing emotional, physical and medical concerns appropriately and reporting safeguarding concerns or neglect.
This does NOT mean that teachers should do anything and everything a parent would do and certainly not without the appropriate training and supervision.
The OP needs to check what is in her contract, the school policy around intimate care, and that there has been a safeguarding risk assessment.
If there isn't anybody able to provide that care then the school (not the individual staff members) are failing in their duty to provide adequate safeguarding.
“Governing bodies should ensure that staff have received suitable training and are competent before they take on responsibility to support children with medical conditions.”
— DfE: Supporting Pupils at School with Medical Conditions, 2015
This extends to toileting needs, as it's highly likely that children past the age of 6 with both bladder or bowel toileting needs will be related to a diagnosed or undiagnosed medical condition.
In this instance, it sounds like the OP has confirmed that the school have 1 staff member that is trained and willing to provide intimate care, but the school have not sourced an appropriate contingency plan for when that staff member is unavailable, and this could be seen as a legitimate safeguarding failure of the school.
Contingency planning is a part of risk management and safeguarding, and it should not fall to untrained, unconsenting staff to fill the gaps. The school should be sourcing adequate emergency cover.
The OP needs to counter this complaint, and has good grounds to do so because she is not trained, has not consented, has not been involved in any risk assessments and has a duty of care to the children to raise this as a safeguarding matter.