From the Guidelines

Influenza B treatment should be initiated with oral oseltamivir (Tamiflu) as the antiviral drug of choice.

Oseltamivir (Tamiflu): The American Academy of Pediatrics (AAP) recommends oseltamivir as the first-line treatment for influenza B, due to its cumulative experience, relative cost, and ease of administration 1.
Zanamivir (Relenza): Inhaled zanamivir is an acceptable alternative for patients 7 years and older who do not have chronic respiratory disease, but its use may be limited by the ability of children to correctly use the inhaler 1.
Peramivir (Rapivab): A single dose of intravenous peramivir is approved for the treatment of acute uncomplicated influenza in ambulatory children 6 months and older, but its efficacy in patients with serious influenza requiring hospitalization has not been established 1.
Baloxavir marboxil (Xofluza): Baloxavir is approved for treatment of acute uncomplicated influenza in otherwise healthy adults, and some studies suggest it may have better efficacy than oseltamivir in the treatment of influenza B, but its use in children is limited by the availability of the oral suspension formulation 1.

Antiviral resistance: The AAP recommends verifying susceptibility data at the start of the influenza season and monitoring it throughout the season, as resistance characteristics can change rapidly 1.
High-risk patients: Patients at higher risk of influenza complications, such as those with chronic pulmonary or cardiovascular disease, should be prioritized for antiviral treatment 1.
Treatment timing: Antiviral treatment should be initiated within 2 days of illness onset to be most effective 1.

From the Research

Antiviral agents such as zanamivir and oseltamivir are indicated for the treatment of both influenza A and B, and must be used within 48 hours of the onset of symptoms to be effective 2
Oseltamivir has been shown to have significant antiviral activity in experimental human influenza B virus infection when used for prophylaxis or early treatment, and is well tolerated with no emergence of drug-resistant variants detected 3
Baloxavir marboxil is a new class of antiviral drug that targets the endonuclease function of the viral PA polymerase subunit and prevents the transcription of viral mRNA, and has been shown to be effective in treating influenza A or B virus infection with just a single dose 4

Neuraminidase inhibitors (NAIs) such as oseltamivir have been shown to be effective in treating influenza B virus infection, but the development of resistance is a concern 5
Oseltamivir has been shown to reduce the duration of viral shedding and the intensity of influenza symptoms in pediatric patients with influenza B, and is well tolerated with no deaths reported 6

Oseltamivir has been shown to be clinically effective and well tolerated in unvaccinated pediatric influenza patients, including those with influenza B, and reduces the intensity of influenza symptoms and the number of days of hospitalization 6
Comorbidities such as asthma, gastrointestinal diseases, and metabolic and endocrine diseases have been identified as associated with prolonged duration of hospitalization in pediatric patients with influenza B treated with oseltamivir 6