Thank you for your query,
The blister is looking like an aphtous ulcer.
Chickenpox will not present with oral lesions.
Hand foot mouth disease will present with oral ulcers but the lesions of hand and feet may be present.
As your child has been exposed to chickenpox consult your Pediatrician
for prophylactic chickenpox vaccination
advise if your child did not vaccinated for it or did not developed chicken pox
For the bump you can apply topical benzocaine
gel if there is pain.
Hope I have answered your query.