Suppose the 14 year old sustained unknown internal injuries, which later cause his death? Without a police report on file, it will be more difficult to prosecute the father for murdering his own son.
A report will also prevent civil actions against you by the family, for not reporting the crime.
That is why, providing that you or a reliable individual observed this child abuse, you should file a formal police report. Indicate you are concerned that the 14 year old might have sustained hidden internal injuries due to the terrible fall down the stairs and subsequent beating; that you want the child abuse documented should the boy later develop complications that require criminal charges and/or civil action by the family against the father.
Also, getting a criminal conviction for assaulting a child will effectively "mark" the abuser for life. I don't think there are any statute of limitations on the number of years a police record exists for a crime.
Derrick
To see a World in a Grain of Sand
And a Heaven in a Wild Flower,
Hold Infinity in the palm of your hand
And Eternity in an hour.
-- William Blake (Auguries of Innocence)