coming of age (?) movie about a boy and a blonde girl

The movie is about a boy who befriends a girl with curly blonde hair and they both enter some other world or dimension I believe?? The boy’s grandfather was important to the story too I think.
The whole plot revolves around this tightrope in the other dimension and the kids were to scared to cross it. I think this was a metaphor for life and death as the girl and the grandfather were sick (if you fall off the tightrope you die, if you cross it you leave the dimension and enter the normal world)
If this helps, one specific scene I remember is when they visit the boy’s bedroom in the other dimension, which is a copy of his real bedroom. The girl and the boy discover that they both like astronomy and talk about the moon and space missions or something. The boy specifically talked about “The Man on The Moon” which had something to do with the father.

Anyway I would really appreciate it if someone knows about this movie and can tell me. Sorry for the long ask and thank you in advance !!