Short influenza postexposure prophylaxis (PEP) showed high efficacy in adults, but studies in children are lacking. This randomized open-label pilot trial aimed to verify noninferiority of a 3- versus 7-day prophylaxis with oral oseltamivir in hospitalized children. Influenza contacts were randomized to the 3- or 7-day group and efficacy, relative risk of adverse events (AEs), and the cumulative costs of drugs and AEs management were compared. The intention-to-treat (ITT) analysis included 59 children (n?=?28 and n?=?31 in the 3- and 7-day group, respectively). The efficacy was 100% (95% CI 87.7–100%) versus 93.6% (95% CI 78.6–99.2%) in the 3- and 7-day group; the differences were statistically insignificant. A per-protocol (PP) analysis including 56 patients (n?=?27 and n?=?29, respectively) showed 100% (95% CI 87.2–100%) and 93.1% (95% CI 77.2–99.2%) efficacy, respectively, without statistical significance. Differences were within the predefined noninferiority margin with an efficacy difference Δ?=?6.45 percentage points (p.p.) with 1-sided 95% CI (? 2.8, ? 1.31, p?=?0.86; ITT) and Δ?=?6.9 p.p. (1-sided 95% CI ? 2.83, ? 1.27, p?=?0.85; PP). Adverse events did not differ significantly, while the cumulative costs of the prophylaxis and AEs management were higher in the 7-day group (median 10.5 euro vs. 4.5 euro, p?