All my children are multilingual. My first born, now seven years old, speaks, reads, writes (all fluently) in three languages (Polish, English, and French). My second is not far behind, he can speak both Polish and English and is now in French immersion kindergarten learning French. My third is just learning to speak, but he knows both Polish and English.
I am Polish, born in Poland. When we came to Canada, I was only six years old. I didn't know how to speak English, and my grade one teachers thought I was mute, because I didn't say a word that whole first year. Finally, I broke my silence in grade two, and by grade five I had no accent in my English Language. "Kids learn languages very easily." - that's what most people think. In my case, this was true. By grade seven, my parents realized that it wasn't too late for me to learn yet another language, so they sent me to French Immersion. Again, learning French in Grade 7 was easy - I was operational by Christmas, and by the end of grade 7 I was fluent.
This is a pretty typical scenario for immigrant children. The only difference is that most of them lose their first language very soon after they arrive in their new home. Their parents don't keep a watchful eye and the language is lost.