In such cases, Few important things to notice and inform would be... Whether the pain is triggered during mastication(after/during). Duration of pain.. and if the pain is nocturnal and also if the child had a disturbed sleep. These answers along with a detailed clinical evaluation will help the doctor diagnose and treat accordingly.
Next Steps
Visit a pediatric dentist to the earlier for evaluation.
Health Tips
Check for ulcers too in and around oral mucosa.