In my view the authorities are allowing her parents to remain in this country while certain processes are under way. If they should be sent back, this should happen now. If there is a reason originating from UK policies to delay this, the UK should ensure the child's welfare in the meantime.
If the family was returned to their home country the welfare of the child becomes the responsibility of that government alongside all of the other children resident there. Sadly, that probably (but by no means certainly) means that this operation would not be available to her.
While she is present in this country through no fault of her own I feel that she is entitled to the same care and support as any other child. She should also be entitled to an education, nutrition, social services intervention, etc.
Were her family to be given leave to remain she would then be entitled to costly treatment and support to help reduce the impact of the disability on her future life. Giving her the operation on the NHS could save system a considerable amount of money.