Parents may want to consider sending their children to one of the reputable English-based summer camps that are held in Japan during the summer. Companies to consider include the following, in alphabetical order.
There are many other English programs available throughout Japan. A Google search should bring up more than are listed here.
(Please note that a mention here does not mean an endorsement from Tsukuba International School of these camp programs. Please do your own research to determine the quality and appropriateness of these camps for your children.)