My family gets these from time to time. TTO doesn't hurt when you put it on the canker sore (thankfully). Another thing to try is red raspberry leaf tea. If your dd doesn't like to drink it, she can swish it and then spit. This really works! I don't remember where I heard about this remedy, but it is the one that works the best IMO. ANd believe me, I've tried them all!
Hope she is feeling better soon.