should i take my child to pediatrician or dentist for a bad canker sore?
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.