Parent of 3.5 year old here. In the US, I would consider socializing your kid in a city then taking two or three 'seasons' (read: years) out to sail around Caribbean countries. These wouldn't need to be consecutive - you could do year on/year off if required. Great language opportunities, lots of nature, good boats are very cheap in Florida, short hops in good weather in the area are perfectly suitable for less experienced sailors, and you are close enough that flying home for 6 months to stay connected to school is a non-issue.