Unfortunately there is not an easy way to extract the compound you need (sodium alginate) from kelp. It is a rather complex procedure. But, that doesn't mean you can't make agar plates on your own.

You can buy agar at most health food stores (it is also called agar-agar) and you can make the plates with soup broth. The trickiest part is sterilizing the plates, which requires a pressure cooker. Pressure cookers can be dangerous and would definitely require that you work with an adult.

Good luck on your project!