I can appreciate that this is a concern. They don't like him even though he's not obviously a protection concern to the children. As you told them, he's abusive and now you're back with him. And you're going to have another child of his.
If I were your lawyer, I'd tell you to tell the CAS that you're pregnant. You are going to have to tell them sooner or later.
Yes, you can tell them. Assure them that you were considering reconciling and resumed sexual relations but remind them that it's over now and you aren't going back to him. It's not ideal that they might conclude that you lied, but they won't apprehend the children just because of this. They will however decide that you need extra support and monitoring with another baby on his way, so you should do your best to not lie to them about anything else, be co-operative, ask for help if you need it, and show that your children are a priority ahead of any relationship with your ex.
As well, now is probably a good time to think about going to court. You say you're co-parenting. I haven't asked you, but I wouldn't be surprised if he thinks that you two are still together but just living apart temporarily until the CAS goes away.
If you're truly done with the guy, then you should go to court for an order for custody, access, and support. Ask CAS if they can help you with this. They won't help you, but you'll demonstrate to them that you're looking for help and that you intend to stay apart from him. Ask them what sort of an order they'd recommend, especially about access.
You might as well wait until after the baby is born until you go to court, since the baby isn't person until it is born. Meaning you can't claim custody until after the birth. If you're getting Ontario Works see if their Family Support Unit will help you with the paperwork. If they won't, the duty counsel or legal aid services at court will help you.
You said that he's not the father of your second child. Maybe you already have an order for custody/access with that father, maybe not. Now, going after that father for custody/support, that's a separate issue. For now you want to demonstrate to the CAS that you and your ex are over and that they don't need to take you to court for a protection order or to apprehend the children.
You should have a chat with your ex about this. He needs to know that you're pregnant and that you're not going back to him and that you have to take him to court for custody in order to get the CAS out of the picture.
If you want to reconcile, then your ex and you have to ask what you will need to do in order to satisfy the CAS that the children aren't in need of protection. They will want him to do a parenting program, probably some anger management counselling, and couples counselling too for you both.
I see that you asked for a phone call but I don't do those. This is a US based website and it's not a law firm.
That's a lot to read and digest, I know. Let me know when you have. I'll wait for your question or comment. If I've answered you then I"d appreciate a positive service rating please.