Every child is entitled to free education in Japan up to the age of fifteen, whether he or she is Japanese or not. As a foreigner seeking education for your child in Japan your only expenses will be just like any Japanese parent, such as for text books, school uniform etc. You may prefer to enrol your child in one of the international schools to be found in Tokyo, where classes are taught in both English and Japanese. This can be an expensive option, but perhaps preferable to a totally Japanese learning environment. Having said this, children will pick up Japanese a lot more quickly and easily than their parents do.
Here are links to the most popular international schools in Tokyo: