A movie (anime/cartoon) about a boy going to another world because his mother is dying it is a hard movie to remember sorry if you can't figure it out.

Brave Story

Mars needs moms? Possibly?

The main character was a girl
but possibly,.

Oblivion Island
A girl loses a mirror given to her by her late mother. She travels to the land
where all neglected and forgotten things go after they're lost to retrieve it.

-view trailer- (http://www.youtube.com/watch?v=92wEkHF63cc)

If not,. remember anything else? What year / decade did you see it ? ---

Brave Story

It appears dkg answered his/her question. For others a link to IMDb's page for Brave Story: Brave Story (2006) - IMDb (http://www.imdb.com/title/tt0778631/)