A movie that has 2kids in it a brother and a sister and think there are fairies and people don't believe them. I remember a part where the brother gets trapped in a mine or a cave and the dad goes to save him and the fairies help.

Possibly Beings (http://www.imdb.com/title/tt0139060/)

@quinn F thank you so much I have been searching for a long time for this!

You're very welcome.