neither. canker sores (aphthous ulcers) assuming you made the correct diagnosis are self-limiting. they can hurt yet but there is little that can be done to make them go away sooner. some medical and dental professionals will use a steroid medication in a special dental paste (so it will stick to the slippery mouth tissue) but just "no treatment" is also a viable option since these things go away a less than a week. i don't know the age of your child or the location of this canker sore however. watch salty foods that can burn these open mouth sores but otherwise just hang in there.
should i take my child to pediatrician or dentist for a bad canker sore?