Hello and Welcome to ‘Ask A Doctor’ service.
I have reviewed your query and here is my advice.
From your explanation it looks like fever blisters only and it is a viral infection leading to sores in mouth along with fever.
It is Viral Stomatitis
Viral infection is commonly self resolving and can take upto 2 weeks to get better.
You can however consult a Paediatric dentist
and get her evaluated and a blood test
and oral swab culture can be done for confirmation.
If it is viral infection she can be advised numbing gel like Lidocaine
to reduce pain as well as do cool compresses.
A course of antiviral medicines like Acyclovir can also be advised.Paracetamol
can be given to reduce fever.
Give him bland foods and plenty of water along with sufficient rest.
Hope I have answered your query. Let me know if I can assist you further.
Dr. Honey Arora