In this model, learners receive literacy instruction in their native language until they acquire a "threshold" literacy proficiency. Some researchers use age 3 as the age when a child has basic communicative competence in L1 (Kessler, 1984). Children may go through a process of sequential acquisition if they migrate at a young age to a country where a different language is spoken, or if the child exclusively speaks his or her heritage language at home until he/she is immersed in a school setting where instruction is offered in a different language.
The phases children go through during sequential acquisition are less linear than for simultaneous acquisition and can vary greatly among children. Sequential acquisition is a more complex and lengthier process, although there is no indication that non language-delayed children end up less proficient than simultaneous bilinguals, so long as they receive adequate input in both languages.

Definition of multilingualism
Comparing multilingual speakers
Models for native language literacy programs
Sequential model