Hi Mel,Hi Chels --
Sounds like a Brill plan. You could contact the charity and ask premission to use their name (letterhead) etc. when you solicit donations. You might be able to get sponsored for the marquee, tables, chairs, etc. rental, get some food donated for the BBQ, get hay bales lent from a local farmer, and so forth.
I did this kind of letter writing every year for Cub Scout Day Camp -- and it was amazing what people, business, and other organizations would provide.
MM