Hi there,
I'm going to post this here because it might interest others as well. David Green would be a good person to approach with this kind of question--if anyone knows about available beach houses near Tokyo, it's he. You can check out his website at:

www.discoverjapan.co.jp

or email David at info@discoverjapan.co.jp

Also, Shimoda is a popular place with some well-heeled foreign families.

Happy hunting!