In this kind of party it's all about the birthday child's first letter of his/her name. Everyone that are invited should dress up as something that starts with that letter. In this post I'm going to pretend that the child's name is Catherine.
Something that starts on C. For example clown, Cinderella, car, captain, cowboy or an cupcake. Or you could dress up as a person. You don't need to go extreme, you could just buy a shirt with a car on it.
I know that everything is done on the computer these days but I think it's so fun when I get a real invitation on paper, so I definitely recommend that!