Dear Queenie,
My son-in-law is from another country where they speak a different language. He speaks English well enough, but he also talks to their young children in his native tongue and they talk to their parents in both language.
Queenie, I’m worried that they will grow up to be confused about what language to use when talking to people outside the family.—Monolingual
Dear Monolingual,
Do not concern yourself. Young children have little problem becoming fluent in more than one language. Your grandchildren will automatically tend to respond in whatever language the person they are talking to uses, and being bilingual may very well be quite useful to them as they grow up.