You could have a superhero-themed party with everyone dressing up as their favorite comic heroes. Also, decorate the place with comic book covers and serve treats shaped like comic characters.
You could have characters dressed up as superheroes or princesses. Or maybe show a cake with candles that look like they're about to blow out.
You could have guests dress up as their favorite comic strip characters. Decorate the place with old comic strips and have a comic-themed cake.
You could have guests dress up as their favorite caricature characters. Also, have a caricature artist draw guests' portraits as party favors.
You could have comic-themed balloons and banners. Also, get some cutouts of popular comic characters to stick on the walls.
Well, you could include a 'Capture the Flag' game like they have at Camp Half - Blood. It would be exciting for the characters in the fanfiction. Also, for the party decorations, having floating Greek columns made of cardboard and painted to look like marble would be really cool. And don't forget about a 'prophecy - writing' station where guests can create their own prophecies, just like in the books.
You could have car-shaped cookies and cupcakes. Also, decorate the place with cartoon car posters and balloons.
You could have a princess-themed party with a castle backdrop and tiaras for the guests. Or a unicorn party with colorful decorations and unicorn-shaped treats.
You could have a cat-themed cake and decorate the place with cat balloons and posters.
You could have a superhero-themed party. Everyone dresses up as their favorite superhero and you can have activities like comic book trivia.
For a girl's birthday party cartoon theme, think about a candyland theme with sweet cartoon candies and treats. Or a garden party with cartoon flowers and cute bugs. You could also go for a cartoon animal theme like kittens or bunnies.