While there are antiquated laws that are still on the books the odds of what you just posting actually happening is somewheer between slim and none. If an officer sees someone assulting a person you can probabaly bet the abuser will go to the jail no matter what.
As far as a child determining where they want to live you if you can post a link to the LAW that states that a child can determine where they want to live as long as they speak clear english that would go more towards proving your point. Remember that a police officer is NOT a lawyer or a judge and they are there to keep the peace.
As Drew said the officer was there to avoid conflict but that does NOT mean that the dad now gets to keep the child. If the mother wanted to enforce the court order all she has to do is drag the father into court where he coudl be held in contempt.
If every child was allowed to live with the parent they wanted they would go with the one who let them stay up late at night, didn't force tehm to do their homework, let them play on the computer all day, etc... The court usually does what is best for the child not what the child wants.
Things to consider:
Have you read the forum Rules: http://community.lawyers.com/forums/t/78789.aspx
Dangerous words - "I don't know", "I was told'
Teh = The