this is a wonderful idea. Childhood obesity is one of the greatest concerns, but few have the will to actively do something about it.
Many personal training organizations have specialty education for training kids (check out the NASM web site http://www.nasm.org/yes/). It is my belief, though, that this is education on top of a personal trainer certification which you need to obtain. Such a certification will also enable you to get professional liability insurance. I am sure that you would not do without that.
I wish you the best of luck for your future.