Well, I think your best option might be to check out some of the international schools. On the International Schools in Frankfurt page, there is a list you can start off with that shows the various age groups on offer. These options may be a bit more expensive but they will more than likely operate as English-speaking institutions, depending on your choice of course.
Check out the link above to get started. There are a number of schooling options in Frankfurt so it may take a bit of time! When are you planning on going? The schools may have long waiting lists so it would be best to get started as soon as possible.
Hope this helps a little!