Animated Film About A Bird Who Walked On Air

I may have details wrong, but roughly…

– Animated.
– A film on VHS.
– English
– It starred a girl who could talk, and a boy who was mute.
– There were humanoid rats at the beginning of the film, which seemed to be enemies of the humans.
– The girl had a grandmother who was either left or who died at the start of the film.
– The two children were seeking a bird that walked on air (rather than flying).

Any attempts at helping me resolve this decade plus long question are greatly appreciated.