Mopsey, Ask the children what they would like to get their father for his birthday. If I were you, I would get him a nice card, nothing mushy and leave it at that.
Your "C" sounds like he's a good one. He is a safe place for your children to go and talk to someone. They need this sounding board more now than ever.
As for your h, whenever he calls, be pleasant, but don't allow him to rattle your cage. Oh, yes, he's still in the punishing mode, but when he discovers that you aren't taking his bait and worrying about his behavior, he'll stop it. As for that phone call, your h is checking in w/home. He's very paranoid right now and wants to ensure that you are home as well.
Mopsey, he's a tough nut to crack, but they all can and will crack at some point. You are a good mother and never let anyone tell you differently. Continue to do what you are doing and don't allow your h to have power over the way that you are doing things in your life and w/your children.