Look, I agree that it’s really fucking shit that TA’s would be expected to do this on the shit money they earn, and I agree that there is risk involved. I agree that the onus is on the school to ultimately provide staffing to meet the needs of this child. If OP just wants to say “No” and not give any reason they can do, but we don’t generally get away with conducting ourselves like that in employment. The school also needs to provide information and reassurance to involved staff about the extent of their personal liability, vicarious liability, insurance issues, etc.
I also think it’s fucking shit that any children develop diabetes, and that living with a chronic condition has long lasting health, educational and psychosocial implications for them. The child deserves an education and as normal a childhood as possible because they’re already on the back foot there with diabetes and SEN.
It’s also shit that this child’s parents have already probably fought tooth and nail to get her into an appropriate setting whilst maintaining employment, family finances, their own mental health, the wellbeing of any other children in the family, etc etc.
Appropriate training from professionals would help massively in mitigating risk, and also in building staff confidence and competence to minimise the risk in the first place. Other measures can also be put in place to mitigate risk, as mentioned above two person checks before administration of insulin, a clear and specific protocol of what to do in every eventuality - including if a mistake is made with too much insulin being given. The child should also have prescribed Glucagon injection kept on site which can be administered in a hypo where they’re unconscious or unable to swallow food. If a mistake was made with insulin administration then appropriate action would be to call 999 immediately, who would provide further guidance until a crew arrived on scene to give further glucagon or IV Glucose and blue light them in to ED. If you gave too much, they don’t drop dead right in front of your eyes, there’s time to get help.
But careful and diligent administration of insulin, checked by two members of staff against a defined plan written by a paediatric endocrinology consultant is unlikely to ever get as far as needing 999/glucagon because of accidental drug error.