Dee Dee,
when we do out Xmas shoots, I only charge a product fee (prints or merchandise) and no sitting fee since the venue is essentially letting me set up there for "free".

I give the owner of the venue 10% of my sales or, if they prefer, donate it to their specified charity.

Keep in mind that most folks only want a 4X6 print in a folder so that should be your "primary" product.

Jim