The lesion described by you suggests candida dermatitis
along with cellulitis
may be evolving into an abscess.
In candida dermatitis there is erythematous rash
with white satellite lesions which is common in perianal area due to presence of moisture.
Mass with boil looking spot may be cellulitis with induration and a pus point.These lesions may be source of infection causing fever.
All these are my presumptive diagnosis on the basis of your description.
I would suggest you to consult a pediatrician
for clinical examination, diagnosis and management accordingly.