First off, that kid is awesome. Its amazing when children, even as young as 7, truly care about doing good things in the world. Theres too many stupid kids stuck inside playing video games these days that only care about themselves. Its amazing they even understand that other people actually exist.
I think your biggest issue is going to be finding a venue. I would try non-profit places such as a church. They always have big parking lots, helpful people, and the desire to do good. Next up is actually getting people to come.
A meet is different than a car show, so advertising or atleast making sure pople show up is the key. Charge $10 or so per entry and try to find people to donate prizes to cut down on expenses. Charge $3 or so for spectators, and give everyone a voting card when they enter. Allow spectators to vote for their favorite car, so no judging BS covers up the true reason for the show.
Contact radio stations, new stations, and other mediua outlets for advertising. They will generally do non-profit things for the cheaps/free if you pull some strings. Who knows, you might even encourage some big time donations, etc. See if the foundation would even help with everything. Im sure they wouldnt mind bringing some children out to enjoy the show and help remind people what their donations are helping cure.
It shouldnt be too big of an ordeal if you can find a place to host it. Everything else should be common sense/planning.
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