I may have details wrong, but roughly…
– A film on VHS.
– 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.