YANBU to feel the way you do. No mum wants her daughter, who is undergoing surgery, to feel unnecessary anxiety, whatever age they are.
If it is a blanket hospital policy, it may be difficult to get the hospital to make an exception, so having some coping strategies will be useful.
For example, DD could ask if she can video call Mum from her phone with WhatsApp at any time. She can ask the staff about mobile signal throughout the hospital, and for the WiFi login in advance. And reassure DD that you will book time off work to support her whether that’s in person or virtually.
If it will help her to know as much detail about the prep and post operative periods as possible, she should ask for this. If hearing this kind of detail would make her feel worse, she should tell the hospital in advance.
Another possibly is that your DD requests, due to her high anxiety, that her operation is scheduled as early in the day as possible, so she is not waiting as long in a fasted state. She could also request early sedation so she can doze whilst waiting. I think she’s unlikely to be given this, but the fact that she’s asking may underline how concerned she is.
This is also a situation where self hypnosis might be helpful. It isn’t for everyone but if she has time beforehand, a few sessions with a qualified hypnotherapist in advance of her surgery day could be a useful thing to try: https://www.hypnotherapists.org.uk/therapist-finder/
Finally, is there any possibility DD could be neurodiverse? If there is, even if she doesn’t have a formal diagnosis, having you physically present could be a reasonable adjustment.
High levels of anxiety can affect surgical outcomes, so her team must be mindful of this and manage it
https://www.cambridge.org/core/services/aop-cambridge-core/content/view/C51CEC6B50D656A7B8A9323439578692/S2059866125000068a.pdf/pre-operative-anxiety-and-its-impact-on-surgical-outcomes-a-systematic-review-and-meta-analysis.pdf
I hope your daughter’s surgery goes smoothly.