The lawyer is right. In Brighton, you should relay this information to lawyer Mary Albert-Fritz (an excellent family law lawyer in Brighton - in fact probably the one to whom your husband talked), who will know how to handle both the school and DCFS, as she handles a major portion of the family law matters in Brighton. Tell her I referred you and ask for a brief free consultation - even offer to pay for her time.) Now, you might also look at the following webpage for some guidance: http://www.care2.com/c2c/share/detail/1067953. Such a sign might work, but more likely not for the reasons I note below. The problem you face here is that school officials are apparently required by state law to allow DCFS unfettered access to kids without notice to parents. That is in the law to allow DCFS to protect kids against parental abuse and to get a more honest response with less trauma to the kid than is possible if some other adult is in the room observing due to the embarassment that may cause. In my experience most DCFS agents are incredibly sensitive to these issues and incredibly good at being as low key and calming as they can. I have seen them in action and I have always been extremely impressed with their professionalism. If it a parent is the problem, it would certainly be counterproductive and frankly, wrong and dumb, to notify the parent before talking to the kid as abusive parents would then have the upper hand. We have to trust someone to deal with the kids in such situations, and that is the DCFS. They have a lot of power and while most DCFS agents are very very good, some can seem arrogant because they are often very strict about what they tell anyone outside DCFS, law enforcement, and the Courts. Now, what I suspect, but don't have the time to look up right now, is whether the DCFS has to get a search warrant or Court order first, which would seem proper unless there is an emergency, which might or might not be your case depending on what the "prank" was. So, the lawyer is right, the law is not going to help you because the school's hands are tied by law, as they must be. Knowing perhaps, what the act alleged is, you may want to see if you can locate a DCFS manual to see if it says not to do this for that type of prank. My guess is there are strict guidelines on when and how this can be done, but I also guess they probably followed the guidelines.
So, bottom line, I seriously doubt you are going to be able to "inform them otherwise." I suggest you drop this as a counterproductive waste of time. Instead I would encourage you to realize that it is good the DCFS is actively protecting kids, so unless something improper comes of these interviews, perhaps you should be glad rather than mad. You are welcome to share these comments with Mary Albert-Fritz if you take my recommendation at talk to her. She is quite public minded, so my guess is that unless you take more than a few minutes of her time, she won't even charge you unless you want her to take some action for you.
So far, this is free to you. Until you pay a fee, I am not your lawyer and you are not my client, so you take any free advice at your sole risk. I am licensed in IL, MO, TX and am a Reg. Pat. Atty. so advice in any other jurisdiction is general advice and should be confirmed with an attorney licensed in that jurisdiction.