It sounds like the social worker has caused quite an upheaval in your family. Anytime you have someone from the outside trying to come into the family to look around, there is going to be upset. But it will be even worse in a family with abuse going on.
When there is abuse in the family, the members of the family will bond together even closer. Although you would think there would be total division (and there is on a certain level), the bond is over shame and keeping the family secret about the abuse. Poppy and Sam have grown up their whole lives with Dave abusing them. His abuse has instilled fear and shame within both of them. So when the social worker tries to get them to talk about the family secret, they get upset and refuse. Part of that is the shame, and part of that is the trauma of having to talk about it and therefore having to face it. Poppy already has issues with seeing the reality of it and with protecting Dave. Sam also has symptoms, including anger issues, and may refuse just based on his own feelings about it.
The kids are not angry at you. They learned from Dave that when something goes wrong to point the finger at you. They are mimicking the behavior they understand. You have been the kicking dog of your marriage all of the kids' lives. It's all they know. Not accepting Dave's abuse would address that issue. But since there are co dependency issues involved, standing up for yourself is not easy right now. So by default, you get the blame. In reality, the kids realize what is going on but feel powerless to stop it so they follow the family dynamic of blaming you, just as Dave does.
How this all turns out depends very much on how you want it to turn out. You are in charge. You are the breadwinner, the parent and the person with all the control. How you choose to use it depends on you. You can give it to Dave and leave the family yourself or you can use what you have and get him to leave. Poppy and Sam will deal with either situation. They may not like either one, but they will cope.
You may need some time to step away from all of this and get perspective. Are you seeing Adele this week? Talking to her can help. Also, if you feel that any more questions from the social worker would not help, you can always put a stop to the inquiry. It would be good to have them involved and helping, but if you do not feel you would accept any of the assistance they offer, then it may not be worth it. Can you also talk with your sister or your friend Cathy? Talking this out with someone will help you find support and clear your mind. I am here as well anytime you need to talk.
Good night Rose. This will pass and things will settle. It just takes time.