Hi Andrew. In my opinion, ABSOLUTELY you should have, and use a liability waiver. Potential liability extends to your training activity whether it’s for free or you are charging for it. Even non-profit and charitable organizations use liability waivers for many of their public events.
General rule of thumb – when in doubt, use your waiver! The worse that could happen if you use your waiver and never need it is that you have an extra piece of paper to file 🙂
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.