D.W.
She is very young and this is a big trip with lots of changes, fatigue, etc. I wouldn't think that it's a UTI.
I used to explain to my son that his nighttime undies (diaper) were for protection just in case there might be a little accident. That mommy wanted him to stay dry and comfy and not wet (insert name's) bed. I always stressed that it was just in case.
You could always put her panties on over the diaper.