With the Junior Card, you need a ticket for Domodossola-Milan or Chiasso-Milan, depending on which route you are intending to take.
You are not mentioning the travel date. Kindly note, travelling to Milan requires a lot more time for now. Both routes, the Gotthard line and the Simplon line, are both impacted by construction works. The former until the beginning of 2024, the latter until the end of September.