Will DCFS consider this abuse?

[equal_rights]

My son recently told me that his "mommy" hit him in the face right after he got off the phone with me because he had told me about a car accident that him and his mother were in.  I tried to file a police report but they said I couldn't because there was no visible bruise, the incident happened 3 days ago.  He said that 9 times out of 10 a child that age is telling the truth but that judges don't take testimony from 3 and 4 year olds so there was nothing I could do.  If I call DCFS will they do something about this?  My son should not get wacked in the face because he tells me things that involve him.