I would like to add (I am NOT an attorney), that the law works somewhat the same in regards to domestic violence. If the police get called, and the alleged offender sticks around, then both will get interviewed and someone arrested. If it is a lower level, minor injury, and the offender leaves the scene before the police arrives, alleged victim gets interviewed and that is it. No charges are pressed, nothing happens.
Child Welfare Services also works the same. If they get a lower risk level report, with say a 7 day response time, and they go to the home but are unable to contact either parent by phone or home visit, the case closes. If the parent is home, they will often force the parent in low level cases to sign a “voluntary contract.” Usually, they threaten to pull the kids if the parents are not cooperative with this “voluntary contract.” However, once the parent signs, now they HAVE to follow through, because they have signed the contract. If they don’t, then it is really an unpleasant situation for them.