On the first trip back to Jakarta after only 9 months he couldn't converse with his grandparents very well, after that, he was embarrassed and he tried harder to speak Indonesian when we got back home. My wife speaks to him 90% of the time in Indonesian, but he answers in English. This past summer he spent 2 months with his grandparents in Jakarta and got back almost to his pre-move ability. Since then he has worked harder to keep it up, he has also started a daily formal lesson plan from my wife. We also have the benefit of having a very diverse population, I would estimate that 40% of his classmates are bilingual which makes it easier for the kids not to feel the pressure to speak English all of the time as they see their friends speaking in other languages with their parents.
This.
I speak in English with my oldest daughter, my wife speaks in Indonesian with her. She responds to her in English, though, because everyone else around her speaks in English. Last time she went back, she would only speak to me in Indonesian, which is fine but pretty much suggests to me that unless both parents are native speakers in the foreign tongue the child will only get about half the benefit.
I am tempted to have home be exclusively Indonesian to see if it will work now that her English is excellent. My understanding is that we've been doing the right way.
