Absolutely terrified of flying here too. I cry on take off, I cry on landing (mainly with relief but also I hate coming down to land), I am unable to sleep and am on high alert throughout. I have to do a 12 hour flight soon and I'm not quite sure how I'll manage.
Anyway, I've tried diazepam but it doesn't stop the fear and panic because it is the adrenalin rush that makes the panic symptoms so bad. Instead my GP has prescribed beta blockers, which reduce the heart rate and block the effects of adrenaline, so the physical response to a panic situation are reduced. So, whilst I'm still scared I'm not also suffering the racing heart, the sweats etc. I am not at all sedated. I'm not against giving medication per se but a sedative seems quite extreme, so I thought I'd suggest it as another possible option.
Also, BA do this book Flying With Confidence which is an easy read, and is also available as an audiobook. It certainly helped me to understand the physics of flying.
Also there are some great hypnosis apps out there. Check out some of Andrew Johnson's apps here: Enjoy Flying app
I reckon your DD can read or listen to the book before Saturday, and she can also listen to a podcast or two to help her quickly learn some calming techniques. See if you can get her to do that and then chat to her on Friday night and make a decision.
Lastly, you're not a bad mum - I think you've been given a tough time on here. If I were to refuse to fly it would have a big impact on our family, and it is important to me to try and tackle my fear so that we can have lovely holidays together. However, I am an adult and I can make my own decisions. I pity you both having this difficult situation. I wish you luck in finding a way to help her.