Whenever possible, try to spend at least a little one-on-one time with each child. This can be something as simple as reading a quick story, playing a game, or taking a trip to the grocery store together. Make this a special time to practice language skills. When older children go to school, make the alone time especially language rich for younger children.
Try to use specific and descriptive words with your children instead of general all-purpose words like thing and do. Elaborate (and repeat).
Don’t worry about holding younger and older children to exactly the same standards. If the younger child is slower at developing, research shows that they usually catch up.
If older siblings take on the role of translators for their younger brothers or sisters, encourage them to let the younger children speak for themselves. But also encourage them in their role as a language tutor.
Trust your instincts. You know your children. If one does seem to be progressing significantly more slowly than you think is right, ask a language development professional (like a speech-language pathologist at a local elementary school) about it.”
Source: King, K., & Mackey, A. (2007). The bilingual edge: Why, when, and how to teach your child a second language. New York: Collins