What was Jem's "betrayal" and what does it show about his relationship with Dill and Scout?

Jem's betrayal was the fact he went to his father about Dill's presence (hiding) in their home. This betrayal illustrates Jem's ever growing maturity, and his need to accept responsibility and do the right thing. Dill and Scout are no longer playmates..... Jem has risen to the occasion and understands the need to look out for them.