Most likely, there is an established culture of mathematical circles in your country or even city; you should get in touch with your local "branch" for ideas about the scope and curriculum for your course. You might want to include your country/city in your question to get more localized advice.

There are many online resources, too, like Art of Problem Solving.

Problem solving and olympiad problems usually works very well, while teaching a university level course on, say, complex analysis early usually doesn't. The mathematics should be elementary and easily accessible (good: elementary number theory, graphs, polynomials in one variable, combinatorics, Euclidean geometry; bad: differential calculus, advanced algebra) and the focus should be on the students doing mathematics instead of listening to it.