You can seek an opinion from a pedodontist or a dentist who is familiar with paediatric dental care. Your description could be an eruption cyst.
Next Steps
You can locate using practo, alternatively you can opt for an online consult using practo.
Health Tips
Kenacort is sometimes prescribed for an ulcer (adult), since you specifically mention that it's not an ulcer, it would be best advised to seek an opinion before applying the same.