Which City are you in?
We - a bunch of friends and I - are working with a couple of charitable groups , and i kow what you mean about them being a little wary. We got those "looks" as well, initially - like we were a bunch of silly bored housewives who were doing this for a lark!.
What can I say - we just persevered and now are definitely part of the teams.
So - if you really want o make a diff. just show them. ALl the NGOs are terribly short-staffed and need as many volunteers as they can get - so long as you show committment. - G'luck