@Heather It's in June so a couple of months! I may see if we can plan a trial run before then. A special treat sounds like a good idea too!
I would suggest who ever is looking after her to take her to a soft play centre to really tire her out for those nights, she would have a really good time there (as most kids do), then after give her something to eat (that she really likes, just for those two nights) so by the time it’s bed time, she will be so tired that she wouldn’t cry as much, she would hopefully just fall asleep.
How long do you have to prepare? Is there time to do a trial run? A blanket than smells like you can help. Also if she has a busy day and lots of fresh air etc hopefully she will be so tired she will just be happy to go to sleep at bedtime. Maybe a special treat she doesn't usually get so she sees the perks of staying with her Grandad?