The best children's parties usually revolve around a party theme. A kid party theme doesn't have to be complicated... you can choose a theme based on one of your child's interests, favorite cartoon characters, favorite books, etc.
You can also choose a theme based on things like seasonal events, a special family event, holidays, a planned vacation, back to school, a new movie, costumes, etc.
Basically, anything that you can plan your party around and have fun with can be a kid party theme. The key is to use your imagination.
The great thing about choosing to plan a kid's party around a specific theme is that it will help spark your imagination when it comes to decorating, planning activities, invitations, food, etc.
Check out some of the kid party theme ideas below for ideas on planning a theme for your own party. You don't have to stick with everything we have listed for a theme -- don't be afraid to mix and match ideas to create your own unique party idea.
The party ideas below are separated into age group to make things easier. Some of the themes can be used for more than one age group, so keep that in mind as you check over the theme ideas.

One Year Old Party Ideas
Blue's Clues First Birthday
Care Bears Party Theme
Pooh's First Birthday
Sesame Street
Party Ideas for Younger Children (ages 2-7)
Pre-Teen Party Ideas (ages 8-12)
Teen Party Ideas
Black Light Party
Harry Potter Theme Party
The Lord of the Rings Theme
Murder Mystery Party Theme
Yu-Gi-Oh Party Theme
Holiday Party Ideas