* Quand je serai grand(e), je veux être enseignant(e). This is a common and straightforward translation. `grand(e)` means "grown-up" and uses the masculine ("grand") or feminine ("grande") form depending on your gender. `enseignant(e)` is the teacher, and also uses the masculine or feminine form.
* Quand je serai plus âgé(e), je veux enseigner. This translates to "When I'm older, I want to teach." `plus âgé(e)` means "older," again with masculine/feminine forms. `enseigner` is the verb "to teach." This is slightly more formal.
* Je veux devenir enseignant(e) quand je serai adulte. This means "I want to become a teacher when I'm an adult." `devenir` means "to become," and `adulte` means "adult." This is a more formal option.
* Mon souhait est de devenir enseignant(e) plus tard. This translates to "My wish is to become a teacher later." This is a more sophisticated and less direct way of expressing the desire.
The best option depends on context. For a child speaking, the first option is perfectly suitable. For a more formal setting, the third or fourth options might be better choices.