Our youngest is slowly getting it, but he takes great offense to any help whatsoever. If we catch him going on the toilet and offer praise we are met with a loud "NOOOOO" then the door is quickly closed. He values his privacy I guess.
I get this from my three year old.....lol.....
When i trained my daughter for the night time it was easy as i would be getting home from work aroud 11/12 at night so i would wake her up and take her, this was great and did it over a fornight, we still had the occasional accident but had an extra sheet and protector under the top ones so no remaking the bed of a night.
But now with my son its a bit harder as the nights are COLD and i guess i haven't really tried hard enough but he had gotten up a few mornings recently with a dry nappy.
Pull ups are a great idea but yes they are expensive and a waste of mony my son thinks they are a nappy and still just goes to the toilet in them.
You may have to talk to the g/parents with your concern and look up so info that may help them with taking her to the toilet.
BTW what time do they go to bed around 9/10 pm if so they may be able to take her before they go to bed(depending what time your daughter goes to bed) and if they are early risers may have to wake your daughter up then and take here. Just another option.
Helen