I see that the original request for help in tutoring native English-speaking children is quite old. But, for those of you in this predicament currently, here's how I handled it.
My daughter came over at age 4. Her spoken English was excellent but she didn't have reading or writing skills in English. I knew that once she was exposed to the systematic rules of German, she'd find English too problematic to try to learn. So my tactic was to start her off in English for a couple of years until the Swiss schools really started emphasizing reading and writing (about 2nd grade).
What I did was to import the most fun workbooks that I could find. I still have the whole series at home. There are books that are designed with reward stickers and other self-motivating tools. These worked great with me daughter. I encouraged her to work daily on them but she could work at her own pace. We did this for years. As a result, she learned to read in English. When, at age 11, she finally switched over to the international school, it took her less than a year to get her spelling up to par.
If anyone is interested in having their kid(s) work with this system, I'd be happy to tutor, individually or in a small group. I now have about five years of experience teaching English as a second language to local Basel children.