I don't have any theories or ideas that haven't already been posted.
I would like to reach out to anyone who has knowledge of what happened to Lucas, or where he may be hidden. If you are reading this, please think of this innocent child before you dismiss his worth to protect your friend.
If you heard that it was an accident, or a natural death, please realize how important it is to dignify Lucas, and help bring him home for a proper burial. Don't leave this child alone and abandoned.
If you know that Emily was responsible for his death and say nothing, you will have to live with this for the rest of your life. Your loyalty to her at this this time, and disregard for justice will haunt you. Are you willing to live with this secret for the rest of your life? You don't deserve this burden. Nobody does.
Look at yourself in a mirror and study your face. Look at your eyes. Think about Lucas and your secret. Be the person who does the right thing.