When I did a surprise party for my sister's 50th BD last year, she was actually involved in the planning. She thought we were having a big party to celebrate all our family BDs from June-September. (We have like 8 or 9 in that time frame) She didn't know till she got there that it was just for her.
I tried to do a surprise for my folks' 50th, and ended up having to tell my mom because she could not, would not leave me alone about what we were doing. It was probably a better decision since she doesn't really do well with surprises.
I think that however it works out will be wonderful and perfect, and will send all my mojo towards keeping them in the dark.