What can girls wear as an outfit on birthday parties?

If the theme for the party is a sleepover one then you'd obviously wear your pj's or you can wear a wanzee. If the theme is popstars then you can dress up as your favourite singer for example, Selena Gomez, Rihanna, Taylor Swift, Cheryl Cole, etc.

If it's a fancy dress party, you can wear a big, pretty gown, or you can wear a really nice dress.

I hope this answers your question thanks.