Vinland Saga, Dívčí Válka Recommendations
Both are set in Europe, during the middle ages (yet, Divci valka starts in Central Europe a few decades prior the early modern period, while Vinland Saga starts in northern Europe).
Main characters are kids (Thorfinn and Sarka) who enroll in the army after experiencing some traumatic event involving their families and their physical integrity (in case of Sarka).
Both kids will follow some experienced mentor with brains for war: Zizka and Askeladd. Their methods are, of course, controversial.
Both kids develope some talent for war: Thorfinn for close range dual wield and knife throwing, and Sarka, with early european firearm.