I commend you for wanting to help the less fortunate.

The unfortunate part is that there really is no complete protection against problems and being sued. It's a shame, but that's the way life is these days. That's why you get an insurance policy in place for your event!
My wife & I have been running a charity event (The Forever Yonge Run) for the past 15 years raising fund for the Foundation Fighting Blindness (FFB). When we started up I questioned my lawyer and asked for him to write up a disclaimer/release that would do the trick. He did, but followed up with "it really doesn't matter how it is written, if something goes wrong, you're at risk."
The cause meant that much to us that we decided to carry on with our plans and trusting in people's good nature. Our event is a ride and overnight stay. Therefore, even though we have a party with dancing & drinking etc. the ride portion is complete by then. The next day we have no planned ride home... Everyone is on their own for that.
In the 15 years we have had some problems with minor accidents during the ride portion of our event (one involved an officer who was on duty assisting with traffic), but fortunately nothing came back to us... Touch wood!
I guess you have to have faith in the fact that people recognize that it IS a charity event, and that they are participating at their own risk. I realize that everyone doesn't always think that way especially when something happens to them and they want some kind of compensation. As mentioned we've been very fortunate.
With running our event so long and having so many repeat participants, we're confident that our participants know the extend we go to in providing a safe environment for them to enjoy themselves while helping others.
The insurance policy is a MUST and well worth the money spent to give you peace of mind.
Not sure if any of this has helped you in making your decision, but just in case and for what it's worth, here is our disclaimer/release that goes on the bottom of every registration form for our event.
The “Forever Yonge Run” organizing committee and/or any of the ride’s sponsors and the FFB assumes no responsibility or liability whatsoever for damages, injuries or accidents of any kind which may occur to any rider and/or their companion while the above signed and/or their companion participates in the ride. The above signed and/or their companion hereby acknowledges and agrees not to hold the Forever Yonge Run organizing committee and/or any of the ride’s sponsors and FFB liable for any and all claims for damages, injuries or accidents which may occur while the above signed participant and/or their companion participates in this event.Good luck with your decision and with your event should you go forward with it.