When? a child is learning a second language?

Code switching/code mixing

Language play

Instructed second language development

nature of young child second language acquisition

L2 acquicition somtimes like&unlike L1's

proclivity of young children to play with the forms and meaning of language

sensitive to the need to move children toward control of the new features of new language

children with multilingual experiences can partially or totally lose or insufficiently develop their first language, even though they do not fully master the second