CGOAB is a facile tool, easy to post to from the road. Neil Gunton, the creator and moderator, is very, very judicious with control issues. I've never seen him step in unless something is genuinely way over the top. Because the target audience is already conditioned to go there, CGOAB is almost certain to gain you a wider audience. Since you're doing a charity ride, the hitcount should trump all other considerations, IMHO. That's what's likely to bring in more support.
Since I work in fund-raising and development, I've naturally got some opinions in this area. I'd contrive to get updates on the children's hospital site, as well as CGOAB. Twitter? Probably more trouble than it's worth.