I would hazard that in those cases, the adults still operate in their own languages as much as they can. The children spend many hours a day in situations where they cannot do so, or where the consequences of language inability are not short-term catastrophic. Schooling, for example. I suspect that in those situations, the adults find ways to continue to operate in their own language, out of necessity; they don't have the luxury of spending several hours a day fumbling their way through basic grammar. Those adults need to work, so they will work where they can continue to use their own language, or at least get by with hand-signals and a hundred words. Often, people moving internationally group together with other immigrants, continuing to operate in their own language and culture. They simply avoid learning.