According to ESRB, this game contains: Mild Cartoon Violence
Parents have almost nothing to worry about with this game. Paper Mario: The Thousand-Year Door has no gore and no bad language. Bowser's lackey Kammy Koopa does say, "I was shaking what my momma gave me!" at one point, but that's the most offensive content I saw. Also, the game requires a lot of reading and very young children may have trouble keeping up.
RPG fans will find a lighthearted send-up of genre conventions while platformer fans can test their reflexes during combat and jump to their hearts' content.
Deaf and Hard of Hearing gamers should have very little trouble playing this game. Early on Mario needs an item that makes a certain noise to defeat a boss, but players can draw on context clues to use it. (I used it quite by accident). Other than this problem, there are no significant auditory cues. Comments during battles (e.g. "Nice!") are written out and the story is told in text.