They say to mark a foul line '4-5 feet' from the bottom. Their pole is only 8' or 9' high tall though, not the normal 10' that you find in schools. The six feet line seems like it is too high for young kids. Yet, I searched a number of sites and found no mention of that height to which you referred. The obscurity of this particular rule makes me think that you should experiment and set your own rule.
Chances are it can vary by age or grade.
Below are a few sites that made reference to informal tetherball rules.