If you are planning to live in Greece, the best thing you can do for your children is to send them to a Greek school. I know of several British families who did this, and their children learned Greek very quickly, and, most importantly, soon integrated into life here, acquired a circle of Greek friends and settled in a way that their parents will never quite achieve - it's a language thing.
There have been a few exceptions.
Teaching your child at home may be an option if you are a foreign resident, I don't think it is if you are Greek. There are British teachers here who can continue your child's British education too. But I feel that it is a great shame to deny your child the chance to become bi-lingual and to learn to be at home in a different culture. It adds a whole new dimension to a child's life that can only benefit him or her as an adult.
As for bi-lingual schools - an attempt was made to open a British School years ago but it fell foul of Greek law.
Catkin's boys are 5 and 7 and I cannot think of a better age at which to start Greek school - 5 yr olds here go to kindergarten anyway as primary school starts at 6 here. Some friends of mine started their kids off at that age and got the most wonderful help from the schoolteacher - it was a small village school, often the best - he helped with extra lessons in Greek. Eight years later, the kids attend a very good school, have done very well, and are bi-lingual.
You will often hear English people criticise Greek schooling. Don't listen. The system lacks the frills that people take for granted in Britain, but on the other hand no child ever seems to leave a Greek school without being able to read or do simple matghs at least. And they learn to be sociable human beings, and to value family life.