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.
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 car-shaped cookies and cupcakes. Also, decorate the place with cartoon car posters and balloons.
You could have a princess-themed party with tiaras and gowns. Or a fairy party with wings and magic wands.
You could have a princess-themed party with everyone dressing up as their favorite princess from cartoons. Or a magical fairy party with glitter and wings!
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.
Well, you could consider a cake decorated with cartoon flowers and hearts to make it super girly. Another option is a cake with a cartoon image of the birthday girl herself having a fun adventure.
You could have a cake with the cartoon girl sitting on a big pile of colorful presents. That would look super cute!
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 characters dressed up as superheroes or princesses. Or maybe show a cake with candles that look like they're about to blow out.