I am not good at this type thing, but I Googled
Cute Halloween birthday greetings
and all kind of sites with Halloween greeting cards came up. I bet you could find something that you could start with on one of the sites.
Just a mention....birthdays that go with holidays get so many mentions of the holiday that people lose track of the fact that it is the individuals birthday, and it just happens to come on a holiday. I have 2 nieces that have birthdays on Christmas day. We make sure each year to separate the birthdate from the holiday. If it were me, I would be focusing more on her event than the holiday and go with something much more traditional regarding this being her 16th which is a very special birthday in itself.