Several of my cousins are to some extent multilingual, due to where they've lived. For English people living abroad, the kids tend to grow up bilingual just because they'll speak one language at school and another at home. One family has now moved back to the UK, and the kids are still very good at languages, though nowhere near multilingual.

What ages did they start learning the other languages, and how much did they speak them? I would guess that makes a huge difference.