That depends on which cartoon characters you are using and in what way. Mickey Mouse for instance is copyrighted by Disney. If you are using these characters as an advertisement or having people wear costumes of the characters during the business hours, you may be risking a lawsuit. If the characters are copyrighted make sure you have permission
I don't think they are concerned about suing for copyright infringement over a kids birthday party. but I'd you are using copyrighted images or characters in a commercial setting in order to make money without the owners consent, then you are breaking the law.