A very similar film was done before in Iran in 1995
A very similar film was done before in Iran. link;
http://www.imdb.com/title/tt0193196/
It is called "Khoda miad" which translates "God will come". It happens in a villager family, the mother is very ill, and everyone in the village lost hope she would recover. The villagers tell the children that only God can help her. One of the daughter writes a letter addressed to God and send it.
This one is not just a simple touching film about innocence, it's a work of art.
I wonder if this film made any references to the Iranian version. Anyway, watch the Iranian version if you got a hold of it.