Kammala jumla mai zuwa: Barnes ya lashe lambar zinare tare da Amurka a 2017 FIBA Under-16 Americas Championship a Formosa, Argentina bayan da ya samu maki 9.8, 3.2 ya sake dawowa da 2.4 sata a kowane wasa. A wasan kusa da karshe da suka yi nasara a kan Argentina, ya jagoranci dukkan masu cin kwallo tare da maki 20 da sata shida yayin da ya karya rikodin Amurka a karkashin 16 don yawan jefa kyauta ta hanyar harbi 8-of-8 daga layin jefa kyauta. A gasar cin kofin duniya ta FIBA ta 2018 a Argentina, Barnes ya samu maki 9.5 da 5.8 a kowane wasa kuma ya sake samun lambar zinare. Ya yi matsakaicin maki 9.7, 4.9 da kuma tallafi 2.7 a kowane wasa a gasar.
2019 FIBA Under-19 World Cup a Heraklion, Girka, inda ya lashe lambar zinare ta uku tare da Amurka.