It is an obligation based on law, equity, and self-preservation. Some of them know neither English nor the first language. Mixing two languages in conversation is a sign of language deficiency. Mixing both languages in conversation is a sign of being bilingual. It is a sign of proficiency in both languages.
All languages change slightly according to the region of origin and social class of the speakers.
Limited English proficient children lack cultural knowledge. There may be some who do not, but the majority do want to learn English, receive and education, apply for higher paying jobs, and fully participate in the life of this country. Limited proficient children have difficulty learning. Some do, but so do many other children who speak English. The only way one learns English is by not using the first language.