This is how it usually works.
ACS will come and make an interview. they will ask as many questions as possible. they will look around the house. might even talk to the children.
The fact that you say that you heard something, does not mean much to them. A lot of people who know the system and abuse their children know how to handle ACS as well. This is unfortunate.
A lot of people who really do not abuse their children had a lot of headaches with ACS, most orthodox yidden get very exited when ACS knocks on their door, they shut the door start shouting.
ACS gets excited as well, what is this person hiding. They are not versed in the mentality of orthodox yidden.
However, if you really think that this is a legitimate case of abuse, I would suggest that you consult with a Torah Authority, so like that you know that even if it turns out that you did more harm than good, you have backing.
Most people do not know, ACS is not interested in keeping your children, they will work together with anyone who is willing to get help and return the children to the parents home.