Traditionally, it has been thought that the ideal way for a child to become bilingual is for one parent always to speak one language to the child, for the other parent always to speak the second language to the child, and for neither parent to mix languages.
(a child; to become; bilingual) (the other parent; always to speak; the second language; to the child)
a child the other parent
to become always to speak
bilingual
the second language to the child