Sci-fi, for sure. The theming of 50's PTC is great, but may be lost on kids (if they don't know who Rob Petrie is, or Wally and the Beav, they might not "get" it, hehe). Sci-Fi is great for sandwiches and desserts - and even if the kids aren't into Sci-Fi the drive-in theme usually gets them.
Good luck!
N.E.D.