Babelmum and the Babelkids are currently visiting the Babelgrands in Algeria.
After a week there, BK1's Arabic had significantly improved. Being in a purely bilingual environment (I was there as well, keeping the German alive!) seems to allow the little ones to concentrate on one language and help them to progress very quickly.
And then the Babelkids played outside and all of a sudden they were switching to English and happily chatting and playing like always.
I was the only one who realised it right away and it made me smile.