Children are hurt by adults of all ages (18+), classes, race, religion and sexual orientation.
These are some reasons physical abuse happens
Family stress, this could be because of financial problems within their household, a breakdown of the parents relationship.
Mental illness can lead to the adult being unable to control their anger feelings an lash out at their child.
Domestic abuse can lead to the parent who is receiving the abuse from some may take out the anger on the child.