Something that i found remarkable. When i was around 12 years old i met two kids who came from Switzerland and who spoke Swiss (the German variant) but by the time i met them they had immersed themselves so totally in English speaking culture they had all but forgotten how to speak their native language.

One of them years later moved back to Switzerland at about age 24, and had to COMPLETELY relearn the language with great difficulty.

So while Children can learn new languages very easily, it does not mean they will store them forever.