**The movie is unclear about whether the Birches will be charged.**

The Birches clearly aided and abetted Keller Dover, so if he is charged, they 
would likely also be charged with some lesser crime(s).

However, given the extreme circumstances in play, it's not certain that even Keller Dover will be charged. When Keller's wife asks Detective Loki if Keller is going to jail, Loki says "probably" but not "yes."

At the end of the movie, the Birches don't seem too concerned about potential charges because they had just got their daughter back and were focused on her.