Because dressing up as superheroes is always a good idea!
Stuck for some inspiration for your child's birthday party?! Why not try a cartoon heroes theme! Every kid loves superheroes (and super villains) and getting everyone to dress up as a character isn't usually too difficult with most having a superhero costume lying around, or at least a shirt with a superhero logo on it!
If you wanted to splash out you could get an entertainer for the children, dressed up as your child's favourite character. Bonus points if this is a surprise, I recently attended a 4th birthday party where none of the kids knew Batman was coming! The look on their faces when he climbed over the back fence was priceless. Not to mention it takes away the pressure from you having to entertain all of the children.
If your budget doesn't accommodate for a children's entertainer why not purchase a costume and dress up yourself! Children will love you put in the effort and there are plenty of simple games that you can play with them to keep them entertained!