Sean is absolutely right. A liability waiver is a must. A:so cover your ground to have permission to use the park. Chances are that the community administration will want to have proof of your liability insurance.
I would also include a photo release clause on the form in the event that you want to snap pictures and post them on Facebook or on your website.
I also include on my forms a note asking people if they want to be included on periodic e-mails (such as the newsletter here on this site) and ask for their e-mail addresses in that case. I am very scrupulous, though, and won’t send anything unless they have checked it off.
Congratulations on launching such a project as a community service.