You can get rid of canker sores by swishing a mixture of warm saltwater and baking soda in your mouth. You should also avoid eating foods that could irritate the canker sores like spicy foods. Canker sores usually heal without treatment within a week or two. Check out www.livestrong.com/article/37155-rid-canker-sore/ for more tips on getting rid of canker sores.
Have the child gargle with a solution of warm water and baking soda. The longer it stays in contact with the canker sore the better. Do this just before bed and by morning it should be better and possibly gone. This can be done 3 times a day if needed.