We all suffer with mouth ulsers at some point in time. They can be quite painful and becomes a major problem in little children when they refuse to eat or drink. They can be caused by

Rare causes include

Most of the time, they can be treated with Antinicrobial mouth washes (Not possible in small children < 5), local anaesthetic gels like Bonjela, LA sprays. Sometimes, they can get infected, needing Antibiotics, but this is rare as long as good oral hygeine measures are followed.

https://www.nhsinform.scot/illnesses-and-conditions/mouth/mouth-ulcer

http://www.bonjela.co.uk/conditions/mouth-ulcers/general-advice/?gclid=CjwKCAjwr-PYBRB8EiwALtjbz85nIcUzcK20sprvCZt7w9Y82Kecw2pWy3cMRayvqe100JWm_9It2xoCkUAQAvD_BwE