On a school field trip I had reponsibility for an 11-year old girl who tended to wet the bed if she didn't take her medication at the right time. All was well until the last night - an evening outing upset her routine and the medication didn't work.
We teachers, and some of the other girls, knew she had problems. When she woke me (too early to get up) I just told her to put on dry things, that it wasn't her fault, and to climb into bed with a friend for half an hour. I stripped the bed and we all settled down for a while. I quietly informed the hostel staff at breakfast and all was dealt with without fuss.
So, Shelflife, as long as the people taking care of your granddaughter know and understand, you don't need to worry about her joining in with things like school trips and Brownie holidays. Her friends might respond better if they're told it's a medical problem but I think most sensible adults will not make a fuss.
By the time she wants to do those things she might well be dry and, if not, able to cope with appropriate underwear at night anyway.