I'm sorry that things are difficult right now. It has been very stressful lately with the social worker involved. A lot of the stress has to do with how everyone in your family is reacting to the social workers involvement. It may help to take a step back and see what you are dealing with.
As I mentioned, everyone is upset about the social worker because she is looking at the one thing they all do not want to deal with, the abuse from Dave. Looking at the abuse equals shame and facing their feelings. In order to do that, they would have to see what they are doing (Dave) and what is happening to them (Sam and Poppy). Even though the kids are being abused, it is the only life they understand. Having someone come in and say "you are being abused, we have to do something about it" upsets them greatly. They do not know what will happen to them if things change.
Also, the social worker visits point out the dysfunction in the family. Dave abuses, the kids are victims, and your role is to be the person who accepts all the fault of the family situation. Upsetting that sends everyone deeper into their roles, just as you are see happening right now.
The key here is how you want to react to all of this. In order to have anything change, someone has to step out of the abuse comfort zone and make a change. You would be the logical choice. The kids can't do anything on their own except react to the situation, and Dave will never see that he is causing all of this. For you to do something, it may be a matter of ignoring your feelings and just doing it. Think of all the things you have learned about your co dependency and your strength these past few months. Now is the time to put them into play and overcome your natural response to accept the blame and not act. Demand change and make it happen. You have me, K and Adele to back you up. You have your family and friends. Everyone is there for you. But only you can start the change. Only you can save the kids and yourself from Dave. The stress of all of this will stop once you decide what you want to do.
I know it's easier to hide from all of this. It feels overwhelming and out of control. But you have more power than you realize. You are the center of the family. You are the strength. The kids will follow what you do. This is your time to leave this part of your life behind and make a better one for you and the kids.