Schadule part of your weekend for Nepali teaching. In weekdays they will be busy with other homeworks. Once they learned Nepali letters, They will try to make word construction same way they learn english in school.  Most important thing is they need to keep practicing every weekend.

Nepali comedy show (Meri bassai, jire khursani, tito satya, Hari bahadur madan bahadur etc) may help to learn some nepali words. It is working on my case at least for now.