I don't have any experience organizing clubs or anything, so I have limited insight here, but...
As you point out, the cost of acquired decent quality bikes and gear is a significant one. Have you tried approaching any brands about providing "sponsored"/discounted equipment? They might be willing to, under the thinking that getting more youth into the sport converts to more adult sales if they stick with it, and the PR for supporting underserved communities. I'm not saying it will be easy, and I can imagine it would take going through some brands declining or ignoring requests before meeting with success, but it's an idea. There may of course be trade-offs like wanting ad banners at the local trails/pump track.
Is there a local trail advocacy group you could try to partner with? They're usually volunteer non-profits, so they may not have much for financial resources to spare and what they do have is mostly spent maintaining the trails, but maybe they could help increase your pull if you do approach more commercial entities?
one or more gifts or other benefits from the co-op.