M.J.
I don't know how that would go over. Most of the older kids I know (10 and up) just do camps during the summer, daycamps, that their friends are in. My park district has already had registration for those programs. So most people I know already have their summer figured out.
To figure out a fee, I would look at your park district brochure and see what their pricing is for a camp and figure an hourly rate. Then I would look at one of their planned trips and see how much they charge for it.
As far as getting the word out, maybe through a church or moms group? Sorry I don't have a better suggestion for that.
M.