This is an old family remedy for canker sores and mouth sores. I recently recommended it to a friend who had horrible mouth sores as a side effect from chemo-therapy, and it was the only thing that gave her any relief.
Go to a larger grocery store and look in the spice section for a spice called alum (or powdered alum). I'm not sure what this is used for in the real world, but I think it has something to do with pickling.
Wet a toothpick and dip it in the alum so there's some alum stuck to the toothpick. Then gently apply a bit of the alum to each sore. It may sting for a second, but probably won't be worse than the pain she's already experiencing. Here's the cool thing about it, especially to kids: it makes you drool like crazy! So make sure she's hanging over a sink when you use it. Then, have her rinse her mouth out with warm water.
We call it "drooly medicine" in our house, and honestly, it takes care of all kinds of mouth sores, usually within 24 hours. When you use it, it feels like it cleans the sore out and cauterizes it or something.